Try Dexron/Mercon
I would try Dexron/Mercon from walmart, is is about $4. 50 a gallon, you will use less than three gallons. You will still have about 2+ gallons of your current ATF+4 or atf+3 in the converter and coolers.
Dexron/Mercon Might-Maybe cause some Lockup shudder, but with a 50/50 mix of new and current ATF, I doubt it. and for the price, if it does shudder, then just drain and fill with the $20. 00 gallon stuff from the dealer.
The lubricating abilities of dexron/mercon are the same as ATF+3/+4, the dexron is stickier, the +3/+4 is slipperier, reducing grabby clutches and shuddering. This is more of a problem with minivans/intrepids/LHS's than with our truck transmissions.
There is a chance of a shudder, but worth the $10-15 gamble IMO.
Hope this helps. Greg L