Just changed the transmission solenoid in my 2008 Dodge Dakota 4x4. It has the 4.7 V8, and (i am pretty sure) the 545rfe transmission. I started refilling the transmission with ATF+3. I have 2 quarts in there. Can I finish off with ATF+4 and be OK? What will likely (not worst possible scenario) happen if I drive around for a month or two with the mix in there?

 

ATF, like brake fluid, coolant, etc., is backwards compatible. You will be fine.

 

ATF+3 is not forward compatible to ATF+4 but +4 fluid is backwards compatible to earlier Chrysler +,+2 and +3 fluids.2005-on used the ATF+4 fluids unless the new +5 fluid is specified.ATF+4 fluid cannot be substituted by any other fluid according to FCA if ATF+4 is specified in the owners manual.

 

Like what daktwos said. Those jugs of ATF+3 must have been very old! ATF+3 is semi synthetic blend. ATF+4 is full synthetic. Viscosities are a bit different too.
No massive panic, world will not end, but you do need to take corrective action immediately.

If you are sitting there right now with those 2 quarts of ATF+3 having just been poured into the dry pan and filters (just installed) and wondering if to proceed topping it off, this is really simple. STOP. Drop the pan and dump it. Reinstall. Fill with entirely ATF+4. Carry on happily ever after.

If you are already done filling it, buttoned up, and have run the engine at all. Then start researching and planning right now on how to properly drain and flush the transmission to displace the ATF+3 and refill with ATF+4 Mopar as soon as possible. In the meantime if you rely on it and have to drive the truck, keep the load and rpm's low as possible. Really baby it. Around town for groceries and beer only type of thing until you can do the fluid swap.