First replacing uppers and not both uppers and lowers is not unusual and is a lot less expensive than both.
Second $500 for the uppers alone is not bad.
Third a 4X4 is harder on the upper ball joints than the lower ball joints due to the stress of the front drive train and additional weight versus a non 4X4.
If the lowers don’t need to be replaced based on the recommendation of your mechanic then save the additional $400 and drive off happy.
BTW, my brother is a mechanic and a set of uppers runs about $800 to $1200 depending on the make and model of vehicle.
OK, thanks. I may not be having this car for much longer, so think I will just get the uppers replaced on both fronts.