There are differences between ATF +3 and ATF +4. Bill Kondolay (of Diesel Transmission Technology) detailed this information in a post recently and on his Q&A BB within his website. If I remember correctly, Bill said the ATF +3 seems to exhibit a little less slippage and the ATF +4 provides better lubrication to bearings etc.
Amsoil's Universal ATF (their latest synthetic ATF offering) is rated ATF +4.
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John Treibel
'98. 5 2500 QC 4x4 Sport, Glasstite Vision II, 285 BFG A/Ts on M/T Challengers, Warn 4X Boards, Mopar fender flares & tow hooks, NW Custom s/s-rubber mud flaps and accs. , Mag-Hytec diff. cover & trans. pan, Edelbrock IAS shocks, Reese Titan V hitch, SPA gauges (3), Dynamat, '00 Sport grille and headlights/PIAA Super White bulbs, PIAA Dual Sport 900 aux. lights, BD exh. brake, Amsoil Dual Remote Bypass, Optima Yellow Tops, DD3s/DD Boost Module, Aeroquip AQP braided s/s fuel hoses, Banks High-Ram intake manifold and 14 cm2 turbine housing w/Big Head wastegate actuator, ATS 3-piece exh. man. , Banks 4" dia. s/s exh. sys. , DTT 93% TC, DTT custom tuned VB, BD modified trans. front pump (enhanced by DTT), Air Bulldog induction hood (with NACA ducts), plus MANY other trick modifications