Our official policy is to take your last CAT about 1 week before test day—and I agree with that. (I wouldn't take a CAT within 5 days of the real thing, FYI.) I think it's also okay for the last one to be two weeks before, if that's what you'd prefer. You might say that you're going to take one about 2 weeks before and, if that one hurts your confidence for some reason, then you can try again 7-10 days before. (If that one also hurts your confidence, then maybe you do actually need to consider postponing.)
In general, I do recommend that you get your brain / body on your "test day schedule" for the entire week before the test—but you can do that even when you have to go to work. Get up at the time that you want to get up on test day, eat meals at the same time, etc. If you have a 4p start, then you will start the quant section around 5p and the verbal section closer to 6:30p, so do a few Q and V questions at those start times. (If you're still at work, step into an empty conference room for 15 minutes or something like that. Don't do a ton—just do a few questions.) You can even plan a couple of work meetings for 4p so that you practice just generally being really alert at the start time. (Just try to be done by 5p so that you can do a couple of quant problems then.)
During that week, if you are able to manage your overall work or non-work commitments so that you are not going overtime or working extra hard, that would be great. Try to avoid setting any deadlines for that week, if possible. Maybe tell your boss that you have to be out the door by a certain time on the W and Th. Tell family / friends / other outside commitments that you're not available after work that week (for anything that you would consider mentally or emotionally draining—but if it's fun, go for it!).
On test day itself, I wouldn't do a bunch of GMAT study or problems—a little high level review of major strategies (including timing!) and rules / formulas is fine, but actual study will just tire you out for later. Anything else, including work or life stuff, is fine as long as it's not very mentally taxing or stressful. It sounds like you might not be going to work that day? If so, then the issue will be how not to drive yourself crazy before 4p.
Plan lunch with a friend who is the type to get your mind off of things or make you laugh. Run some errands. Exercise. Take a short nap (or, if you can't actually fall asleep, then just rest / meditate) around 1p or 2p. (Don't sleep for more than about 30 minutes.) Have something to eat around 3:30p, before you go into the test center—protein, fat, some complex carbs. It doesn't have to be a big meal—but enough to make sure you won't be hungry for a couple of hours. And bring food with you to eat on your breaks.
I'm not sure how far out the date is, but if you aren't going to work that day but you scheduled the 4p time because nothing was available earlier, you can keep checking the website to see whether something opens up (they will charge you to reschedule the time, FYI). It's possible that a slot will open up at 2p or 12p or whenever—people do cancel.
Let us know if you have any other questions. And let us know how it goes!