Gets the service principal auth token using AzureAD module and then get the user and its custom attribute.

Get-AzureADUserOnPremisesExtensionAttributes.ps1

param
(
    [Parameter(Mandatory = $true)][string]$clientId,
    [Parameter(Mandatory = $true)][string]$tenantId,
    [Parameter(Mandatory = $true)][string[]]$upn
)

Install-Module AzureAD
Update-Module AzureAD

$AadModule = Import-Module -Name AzureADPreview -ErrorAction Stop -PassThru

function Get-MSGraphToken
{
    Param([string]$clientId, [string]$tenantId)
    $resourceURI = 'https://graph.microsoft.com/'
    $authority = "https://login.microsoftonline.com/$tenantId"
    $redirectUri = 'https://login.microsoftonline.com/common/oauth2/nativeclient'
    $adal = Join-Path $AadModule.ModuleBase 'Microsoft.IdentityModel.Clients.ActiveDirectory.dll'
    $adalforms = Join-Path $AadModule.ModuleBase 'Microsoft.IdentityModel.Clients.ActiveDirectory.Platform.dll'
    #$scope = 'scope=user.read.all auditLog.read.all directory.read.all'
    [System.Reflection.Assembly]::LoadFrom($adal) | Out-Null
    [System.Reflection.Assembly]::LoadFrom($adalforms) | Out-Null
    $authContext = New-Object 'Microsoft.IdentityModel.Clients.ActiveDirectory.AuthenticationContext' -ArgumentList $authority
    $user = [Microsoft.IdentityModel.Clients.ActiveDirectory.UserIdentifier]::AnyUser
    $platformParameters = New-Object 'Microsoft.IdentityModel.Clients.ActiveDirectory.PlatformParameters' -ArgumentList 'Always'
    $authResult = $authContext.AcquireTokenAsync($resourceURI, $clientId, $redirectUri, $platformParameters)
    $token = $authResult.result.AccessToken
    return $token
}

function Get-User
{
    Param([string]$accessToken, [string[]]$upn)
    foreach ($identity in $upn)
    {
        $uri = "https://graph.microsoft.com/v1.0/users/$identity" + '?$select=id,userPrincipalName,createdDateTime,onPremisesExtensionAttributes';
        $headers = @{
            Authorization = 'Bearer ' + $accessToken
        }
        $response = Invoke-RestMethod -Method Get -Uri $uri -Headers $headers
        $selectedData = $response | select Id, UserPrincipalName, CreatedDateTime, @{Name = 'ExtensionAttribute4'; Expression = { $_.onPremisesExtensionAttributes.extensionAttribute4 } }
        Write-Host $selectedData;
    }
}

function Get-EA4
{
    Param([string]$clientId, [string]$tenantId, [string[]]$upn)
    $accessToken = Get-MSGraphToken -clientId $clientId -tenantId $tenantId
    if (!$accessToken)
    {
        Write-Host 'Missing Token'
        return
    }

    Get-User -accessToken $accessToken -upn $upn
}

#$clientId = '07ea3c4b-c6e7-4fe9-b5eb-8b56ebdd2184';
#$tenantId = 'af038be2-7929-453f-b50d-cd5c2c1c9b87';

Get-EA4 -clientId $clientId -tenantId $tenantId -upn $upn