My husband is planning to make new cabinets for our kitchen and I'm looking at a Kreg Jig for him. What are the differences between the K4, K5 and the Foreman pocket hole machine?
First let's differentiate between the two most similar of the three. The K5 is the successor to the K4, and the main difference is that the K5 moves the handle to the front instead of the back. In other words when working on large workpieces with the K4, you have to reach around to the back side of your workpiece (side opposite the drill guides) in order to clamp the piece in place. On the K5, the drill guides and clamp handle are on the same side, making it a little more convenient to use. The K5 also adds some storage compartment "wings" on the sides of the jig.
For a while, only the K4 was available as a Master System (extra accessories including the Portable Base, Material Support Stop, Premium Face Clamp, and the Dust Collection Attachment). However, as of December 2015, this is no longer the case and there is a K5 Master System. You can think of the K4 as the older one, and the K5 as the "new and improved" jig. The price difference is negligible if you're buying new.
Now let's compare the K5 with the Foreman. With the K5 (and K4, etc.), you use your own drill with a special drill bit provided by Kreg. Like most Kreg pocket hole jigs, the K5 has hardened bushings that guide a drill bit into the wood at the correct angle. If using the included clamping system, you'll most likely want to screw the K5 onto a work platform or base which is clamped onto a table. If not, you'll use a face clamp to clamp it onto your workpiece (which more or less converts it into a Kreg Jig Jr). It requires a little extra work to drill repeat holes, but stores easily. Typical setups orient the jig so your workpiece stands vertically since that's the best orientation of the jig to operate your drill. This isn't to say that you cannot orient the workpiece another way when using the jig's clamping system, but it can be a little more awkward. If you have large workpieces and standing them up vertically isn't practical, recall that you can remove the jig (the part with the drill guides) from the clamping system and use a face clamp or similar to hold it in place.
The Foreman, on the other hand, has a built-in motor, a scale (ruler), tool-free stops, and a large work platform. You clamp your workpiece then pull a lever which automatically turns on the motor and drills your holes. Another difference with the Foreman is that a typical workpiece is oriented flat on the table rather than standing up vertically, since the drill guide is integrated into the table. The Foreman is intended for professional cabinet shops and unless you use pocket hole joinery for almost every project and have a large shop, it probably isn't worth paying 2.5x-4x the price for a tool that does not store as easily.
The K4 is essentially a basic version of the Kreg jig (not the most basic though). The key feature (in my opinion) with the K4 is the rear handle as compared to the front handle in the K5. The front handle is usually preferred by most people for its usability (the K5). The Foreman machine is essentially a self-contained device that does the entire "pocket hole" process without the need for an external power drill. The drill is built into the machine and drills from the bottom rather than the top with most other pocket-hole jigs. The end result is the same with any of the three devices you mentioned.
Besides the handles in the K4 and K5 (excluding other minor details), there isn't much of a difference. The Foreman Machine has the biggest difference between the three. The price is also a huge difference. You are paying for convenience.
The Foreman is probably overkill unless you're basically going into production -- at least a large kitchen's worth of cabinets. It's a bench tool, with its own motor and drilling head. If you are going to be producing many identical pocket-hole joints, it lets you turn them out assembly-line fashion. If the joints will vary in placement and/or in thickness of wood being joined, the Foreman's advantages are wasted and the space it takes up on a worktable is probably not a good investment. And it's more expensive.
But-- I don't know your husband's plans for the tool, I don't know how large the workshop is, and I don't know how far his toy/tool budget will stretch.