For a recipe that calls for a 6 quart crock pot, can I use a four quart crock pot? Should I only use 2/3 of each ingredient?
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.