Hi, I am not too familiar with mark iv, but am a bit surprised the board needs to be removed. If the board is right down is there enough access?
Otherwise with the boat on its side (either afloat in shallow water or on something soft like grass) access to the retaining plate should be possible.
Two people can work the boat gradually off the trolley onto grass – one person lifts the bow while the other person moves the trolley out. Hold the boat careened on its side with the jib or spinny halyard secured to something strong.