I'm going to try to change my transmission fluid in the next week or so, it needs it quite a bit. I've talked to about 5 different transmission shops so far. All of them have said the same thing: Don't get a trans flush. The reason seemed to be the same over and over. Basically, since you're adding brand new fluid, with brand new detergents, all that new soap would scrub off any varnish or gunk in the system and it would clog the filter pretty fast. I guess that's why it's not a bad idea to change the filter right before or right after, then. One also said the pressure of the flush could damage seals.
In any case, they all recommended just regular fluid changes to keep enough good fluid in the system, which sounds like good advice. I've been recommended to use synth blend, even though I wanted to use full synth. It looks like, from a quick google search, that Toyota T-IV is synthetic.