hmm, i doubt that has anything to do with pressure, 4 bar g is a lot (approx. 56 psi), even for a small wheel. with any luck, someone might have one and can advise
if all else fails, check for what seems to be a reasonable tyre compression when fully laden - im guessing the trailer isn't sprung, so it'll probably be better a bit more squishy that you'd expect a car tyre to be...