Ouch an almost 5k lb vehicle with 4 cylinder and automatic!  Don't do it.
To go from std to auto is a royal pain with lots of wiring to swap out.
Absolutely not worth it.

It would probably be easier/more convenient to sell that one (they're quite
desirable, the 5spd 3rd gens, apparently, since my dealer calls us at least
once a year saying they've got a customer begging for one) and buy another
with the automatic transmission. I can't see why anyone would NOT like the
5spd 4cyl set-up, I'm absolutely nuts about mine.

You will need to swap more than the transmission: the ECU, some wiring,
and of course the transfer case, propellor shaft, etc.

Not really advisable; better option would be to sell/trade for the
configuration you want.  And do be aware that the 4cyl/auto combination
is the least powerful.

lycka till!
GTr