Can I use a 4 quart crock pot instead of a 6 quart?
I have encountered very few recipes that can't be scaled. In fact the only recipes I can think of are jelly because the pectin behaves oddly.

If the scaled recipe fits comfortably then you are fine. 

Keep in mind that a smaller volume may cook faster. This shouldn't be a problem for most slow cooker recipes but use your judgement. If you are braising a pot roast then a bit too long won't hurt much. If you are cooking peach cobbler it might need to finish earlier.