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The error code 0x4 in Windows Remote Desktop Protocol (RDP) typically indicates a network-level authentication failure. From my experience administering cross-platform environments, this often manifests when the Remote Desktop client (whether on Mac or Windows) cannot complete the NLA (Network Level Authentication) handshake with the target Windows 10 machine.

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First, check these fundamental settings on the target Windows 10 machine:

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# PowerShell command to verify RDP settings
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>Get-ItemProperty -Path 'HKLM:\System\CurrentControlSet\Control\Terminal Server' -Name 'fDenyTSConnections'
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>Get-ItemProperty -Path 'HKLM:\System\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp' -Name 'UserAuthentication'

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The first value should return 0 (connections allowed) and the second should return 1 (NLA enabled). If not, apply these fixes:

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# Enable RDP and enforce NLA
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>Set-ItemProperty -Path 'HKLM:\System\CurrentControlSet\Control\Terminal Server' -Name 'fDenyTSConnections' -Value 0
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>Set-ItemProperty -Path 'HKLM:\System\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp' -Name 'UserAuthentication' -Value 1
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>Enable-NetFirewallRule -DisplayGroup 'Remote Desktop'

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In hybrid environments (especially with Mac clients), the 0x4 error frequently occurs due to credential provider mismatches. Try adding this registry value:

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Windows Registry Editor Version 5.00
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>[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System]
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>"CredSSP/Parameters"=dword:00000000
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>"AllowEncryptionOracle"=dword:00000002

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When debugging RDP connections, I always recommend:

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1. Capture network traces with Wireshark or Microsoft Message Analyzer

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2. Check Windows Event Viewer logs (Applications and Services Logs > Microsoft > Windows > TerminalServices-*)

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3. Test with different authentication methods using the RDP file parameters:

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authentication level:i:2
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>enablecredsspsupport:i:0
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>negotiate security layer:i:1

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The Mac RDP client (version 10+) enforces stricter certificate validation than Windows clients. To troubleshoot certificate issues:

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# Export the current RDP certificate
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>$cert = Get-ChildItem -Path cert:\LocalMachine\My | Where-Object {$_.Subject -match "TERMSRV"}
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>Export-Certificate -Cert $cert -FilePath C:\temp\rdpcert.cer -Type CERT

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Import this certificate into the Mac's keychain with "Always Trust" settings when connecting.

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For environments requiring legacy support, modify the security layer negotiation:

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# Adjust SSL/TLS settings for RDP
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>New-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Client" -Name "Enabled" -Value 1 -PropertyType DWORD
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>New-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Client" -Name "DisabledByDefault" -Value 0 -PropertyType DWORD

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Remember to restart the target machine after making these changes. For persistent issues, consider using alternative ports or testing with the Windows built-in RDP client rather than third-party implementations.

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When attempting remote connections using Microsoft Remote Desktop Client (version 10) from macOS to Windows 10 or between Windows machines, users frequently encounter:

Error: Your session ended because of an error.
Error code: 0x4
Suggested action: Contact network administrator

This typically indicates authentication protocol mismatches during the NLA (Network Level Authentication) handshake.

The 0x4 error often manifests when:

• CredSSP encryption oracle remediation is enforced
• TLS 1.0/1.1 are disabled on the host
• RDP security layer misconfiguration exists
• Certificate trust chain validation fails

For Windows hosts, modify these registry values (backup first):

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters]
"AllowEncryptionOracle"=dword:00000002

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client]
"DisabledByDefault"=dword:00000000
"Enabled"=dword:00000001

For enterprise environments, apply these Group Policy settings:

Computer Configuration > Administrative Templates > System > Credentials Delegation
- Set "Encryption Oracle Remediation" to "Enabled: Vulnerable"

Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Security
- Set "Require use of specific security layer for RDP connections" to "SSL"
- Set "Require user authentication for remote connections" to "Enabled"

For macOS Remote Desktop Client v10, add these launch parameters:

defaults write com.microsoft.rdc.macos DisableCustomRDGatewayCAPaths -bool YES
defaults write com.microsoft.rdc.macos DisableNLA -bool NO

Test connectivity with PowerShell:

Test-NetConnection -ComputerName [TARGET_IP] -Port 3389
Test-WSMan -ComputerName [TARGET_IP] -Authentication Negotiate

Create proper NSG rules for Azure VMs:

az network nsg rule create \
  --resource-group MyResourceGroup \
  --nsg-name MyNSG \
  --name AllowRDP \
  --priority 100 \
  --source-address-prefixes '*' \
  --source-port-ranges '*' \
  --destination-address-prefixes 'VirtualNetwork' \
  --destination-port-ranges 3389 \
  --access Allow \
  --protocol Tcp \
  --description "Allow RDP"

For testing environments only (not recommended for production):

$RDPCertOverride = @{
  Path = "HKLM:\SOFTWARE\Microsoft\Terminal Server Client"
  Name = "AuthenticationLevelOverride"
  Value = 0
  PropertyType = "DWord"
}
New-ItemProperty @RDPCertOverride -Force