ive read countless threads on this forum alone where rb26dett's from r32gtrs have been rebuilt. the search button is your friend.
$4k is a fair price for the head to be pulled out fixed and refitted. This involves removing the whole motor and stripping pretty much the top half of the motor. A fair amount of work involved.
I had oil in my radiator. This was from a crack inside 1 of the oil galleries in my head. It was not repairable so i needed a new head. I ended up having the whole motor rebuilt with forged pistons and rings. I did this cos i intended on using the car on racetracks and wanted a bit more security. In total, i spent around $14k on my car. This was the motor, powerfc, clutch and a few extra goodies.
If its suspected that there is either a crack in the head or the head gasket, you need to pull it down to find out., Theres no sure fire way to tell until you do pull it down. If you do this, maybe look at what you want the car for. Do you wanna have a nice clean car for road only or do you wanna go to the other end of the scale and go flat out round racetracks every opportunity you have? Then look at what to do from there.
My 2c