you were reading, because they should have made it clear that a small semi-aquatic setup will not work for a softshell.
You did not mention what species of softshell you are aquiring. This can make a huge difference, both literally and in terms of care difficulty. In many species, wind up with a female and you are starting with what will become, size-wise, twice the turtle that a male would have.
Any and all species of softshell will need a truly aquatic setup that is, preferably, strongly filtered, since they don't handle abrupt changes in water pH or chlorine levels any more comfortably than do most fish. A layer of fine gravel will be a big plus, as this will allow the turtle to do what comes very naturally to it - dig into the substrate and remain buried. This activity is also important in keeping the shell clean of debris.
Insofar as where to put the turtle, big tanks are expensive (start with at least a 20 Long-style). It is much easier to get a Rubbermaid or Steralite plastic basin from one of any of the big retail stores, as they are quite cheap and relatively sturdy if you're only filling one up to eight or nine inches. That's more depth than you'd need for a four inch specimen. Also, the accompanying plastic lid can be placed over it for keeping the air warm in the winter, since you'll need a submersible heater. Ebo Jager heaters are the better bet, and can be purchased at drsfostersmith.com, among other places.
Diet. Freeze-dried items are not a diet. They are the crickets and mealworms that are even more devoid of necessary vitamin and mineral contents than the live versions. A nutritionally complete turtle diet such as ReptoMin or Mazuri Freshwater Turtle Diet will work, if the animal will accept it. Some softshells take food sticks, some do not. Should yours fail to, fish will serve as a good diet, and insects can provide variety.
It is imperative that the water remain quite clean. Remember that nitrogenous wastes (ammonia, nitrite, nitrate) are invisible yet can become a substantial threat to animals in a closed environment. Turtles are, gram for gram, among the nastiest creatures in the world.
Don't take this with offence, as I do not mean for you to, but be careful when it comes to those caresheets. It seems a common trend for vets and vet students to rely far too heavily on caresheets when it comes to herps. I can understand why. You guys are confronted with so many different organisms that are in many cases not particularly related (other than being called reptiles and amphibians by means of our language) and usually under-researched insofar as health maladies. Just the same, while there are some very, very good chelonian caresheets out there, I have never known one to come from a pet shop or vet office (although, admittedly, I have been to relatively few of both). Oftentiems, the person who writes a caresheet is reiterating what was written for a "closely related" species, or is basing his or her words off of experience with half a dozen specimens - not a significant number by which to represent any entire species.
These turtles are neat captives. Read up on five books that contain a good deal of natural history information about them, then read articles on preferentials towards big-fish tank filtration. That will be a start. Good luck with your little guy.
Also feeder minnows or guppies (stay away from goldfish for they are high in toxins). At four inches, he/she will also probably feed on thawed pinkies. Hope this helps, and good luck with the softie, they truely are amazing creatures. ~Christy~