Some people may say use DexMerc as a substitute, but do NOT do this unless you know for a FACT that your transmission is slipping, or if it is a dead emergency. If the trans is in good shape and you use the DexMerc, make sure to change the trans fluid ASAP, and only use it to top off of a good trans in a emergency. If the trans is slipping bad, a complete fill of DexMerc will save the transmission. I got an extra 20,000 miles out of my transmission that way...
But use ATF4 on a good transmission, any brand will work, I prefer NAPA's Performance ATF line.