Simple RC circuit with diode question
Basically it's your simulator - it can't ever be higher than the 5V supply and 4.2 is pretty normal. Check that your initial conditions don't include some form of charge on the capacitor. Simulators can throw curved-balls sometimes and you did the right thing by asking.

By the way I think @pjc50 may have misread your voltage as being 9V not 5V - or maybe it's me - the picture isn't all that clear.