$Username = [Environment]::UserName
$Password = $($PassBox1.text)
$Pass = convertTo-SecureString -AsPlainText $Password -Force
$SecureString = $Pass
$MySecureCreds = New-Object -TypeName System.Management.Automation.PSCredential -ArgumentList $Username,$SecureString

$Servers = "Server1","Server2","Server3","Server4"

$Servers | %{Invoke-Command -ComputerName $_ -ScriptBlock{Get-Service -Name "n4cluster"} -Credential $MySecureCreds}
$MessageBox1.AppendText( ($Servers | foreach {$_  + "`n" } ) )