Here is a photo of my Wiha # 1 posidrive screwdriver bit.



Notice the facets are squared off and not sloped and there are four additional tiny facets that relate to the four unique tick marks on a posidrive fastener that relate the screw and driver for posidrive fasteners. In other words, the edges that grip the screw are ground square and not sloped like a philips driver or screw. That is the improvement of the posidrive invention. The tightening or loosening effort doesn't push the posidrive driver out of the fastener.

I don't see any evidence of those four marks on Amervo's example fasteners and I don't recall any such markings on the four I removed.

If you use a square facet posidrive on a #4 philips with ramped facets the results won't be any better than using a number 2 philips on the screw, because the driver won't fit the fastener properly.

Otherwise use vice grips, grinder, fire wrench or weld a hex nut on the end. None of those approaches save the original fastener.