I'm wondering if the PO may have tried to loosen lock nut on valve 4 ? with a wrench a some type of hammer and got up against the head of the bolt and cracked it unkonowingly? Then maybe after time it broke completely off? I actually thought I took the valve cover off for the 1st time because of the badly out valves. Just trying to figure out what caused the bolt head to break off.

Your supposition is plausible, but it could just have been overtightened the last time someone changed the head gasket. If the age and condition of the head bolts are unknown you should buy new ones when you replace the head gasket. Courtesy Parts, MSA, or your local Nissan dealer are good sources for the head bolts.

Be forewarned that the bolt located between the exhaust ports for the #3 and #4 cylinders is very difficult to remove.

By the way, the technical name for the head bolts used on an S30 is "Socket Head Cap Screw". (Not female bolt.) But the dealer will probably just call them head bolts.