The Dsus4 guitar chord is one of those that sounds harder to play than it actually is.
If you know your basic D Major chord then all you need to do is add your little finger to fret 3 on the thinnest string. This will cancel out your middle finger, but you may as well leave that finger held down as lifting it up will only take extra effort.
Dsus4 and Dsus2 are often used as passing (brief) chords when strumming a D major. You can see how it would be easy to take away or add in fingers to a regular D to get a different sound.