why would you jump from 3 plates to 4 plates,if you never tried, 4 plates will crush you, no matter how much you can rep with 3 plates. doesnt work like that. why not try to add 1,25 kg plates every workout or every other workout, that will get you somewhere.
also i noticed something else! when a spotter hands you the weight for the first rep, your body doesnt know how heavy it is, so you get surprised and cant lift one damn rep, destroys the concentration, i like to take the barbell my self from the first rep, that way the mind and body know what its neceserry to lift that heavy weight!!!!