Pump therapy is a wonderful and effective way to control diabetes. With your son being as young as he is, it may seem as if he has been dealt a blow that he doesn't deserve, and I applaud your willingness to treat this life-long commitment head on with a pump.
I know the T-Slim seems to be all the rage, and indeed, it is a nice looking pump. With its touch screen and cool look, it appears to be the latest in technology. What I don't like about the T-Slim is that it must be recharged almost daily, rather than a battery replaced every three to four weeks. Also, the tracking software that keeps and generates reports on your computer for use by your Pediatrician and endocrinologist is not yet available, and there is no due date for it. Also, it is important to consider durability, due to what a four year old will give that pump over the next four years. T-Slim is not yet proven to be reliable.
Both the Mini-Med and the Animas are good pumps. Personally, I use the Animas. I chose it because I liked the colour screen, the remote features with the meter, the food catalog, and its many fail-safe features. The Animas is a very durable pump, and has stood up quite well for me in improving control. It measures in .25 units, gives good reports through two different tracking software choices and the infusion sets don't need a separate inserter. The Animas is completely water proof, and is the only the only pump on the market that is.
The Mini-Med is also a good pump. The Revel series has a built in Continuous Glucose Monitor in it, and also has some remote capabilities, similar to the Animas. It does not have a food catalog. The Mini-Med has good tracking software and is a very durable pump. It does use its own infusion sets, and most of those require the use of a separate inserter.
Were this my choice, I think I would look less at slick features, and more at durability and ease of use in the long run. All the best to you in your decision.