From what i hear, you will need the different computer if you change from a 3 to 4 speed tranny.
Post up a wanted ad in the for sale section that you need a good used auto tranny, and you could probably get one for $200 or less plus shipping.
You might be able to bolt up a 5-speed sidekick tranny behind it, but you'd need the flywheel and clutch and all that.
It would just be best to replace it with the same thing.