Can you make a slow cooker smaller?

I do not see where this would be an issue. You can cook anything in a larger pot. Just put the ingredients in and cook as directed. You may have to check to see it it's done a little sooner, but that's it.

Agree with previous comment. My 6 qt says how to bake a cake (although I've never tried it) and that's WAY, less than 6 qt-so your 3.5/4 qt is no problem! It may get done sooner; so I wouldn't leave it for too long, without checking....

I agree with both the above suggestions as I have done this and the item is cooked a bit quicker, so figure that into your schedule, to check halfway through then at least every half hour there after or less.

Depends on two things, the kind of food you are cooking and the age of your crock pot. Most crock pots manufactured in the last few years heat to a higher temperature. Therefore things will burn more easily in them if you alter recipes that way. On the older models that isn't as much of a concern.

Some things, like soups and stews will cook just fine at a quicker speed, but sometimes since you have less food absorbing the heat the temp gets higher, the food will not gently simmer as intended, but will boil which isn't good for the texture of meats you are trying to tenderize.

Big hunks of meat you can just add a little extra liquid to the pot. Generally if you aren't totally submerging the meat then the liquid needs to come up to at least a third of the way up the side of the roast, so increase liquid and seasonings until you reach that level.

Casseroles, breads, and cakes and such I would put a smaller container in the crock pot to get it to cook without burning unless I was around to watch it. Putting those sorts of recipes in a pot a third as big could alter the cooking time a lot, if it was something I wanted to cook consistently I'd be home the first time to watch it and mark the difference in time in my cookbook. The time would be shorter if you just dumped it in the pot, and a touch longer if you used a container insert.

If you will be unable to check it all the time I would suggest you add some extra fluid as it will evaporate quicker.

Perhaps you might have an oven-proof, microwave-proof dish that would fit inside the crock pot itself. Just a thought.