There's a few arguments back and fourth about that very question. You might need to go to TheRangerStation.com to find the exact answer.
But basically yes the swap from a 4r55e to the 5r55e is possible. You won't however have 5 speeds if you left your current PCM in the truck. The 5th gear in the 5r55e, -2nd to be exact-, is achieved by the PCM telling the O/D band to 'split' the 1-2 shift, essentially adding a gear between the two. I won't be able to explain that to you as I don't exactly know how it works, but that's how it's been explained to me. I think alls you'd need to do is swap the neutral safety switch from the 4r over to the 5r. If you use your stock PCM the 5r's VSS won't be used, and you just would need to plug that hole.
If you wanted the 5speed to be a 5speed you'd have to swap PCM's and a section of the wiring harness that goes back to the trans. Don't quote me but I think you'd also need to use the VSS on the tranny, instead of on the rear diff. ((or it's the other way around, I'm tired... I think our 4r's are on the rear diff.))
Here's a link to a successful swap:
http://www.therangerstation.com/forums/archive/index.php/t-34303.html
EDIT -- basically from all I've read, with the stock PCM, it's a direct bolt on with the exception of a few sensors (VSS and DTR sensors on the 5r) won't be used. And you'd want to get a new EPC solenoid due to pressure differences.
