Statements about the amount of litterboxes to have are based on average findings, including those of cats with known behavoiral elimination issues.
To help prevent any inappropriate elimination habits from forming yes, you should have AT LEAST 1 litter box, per cat, per level of your home. I have 3 cats, I have only one story to my home, I have 3 litter boxes.
For sanitation reasons, specially if you have 4 cats and only a few litter boxes, you should scoop them once or twice a day, not every other day, dirty litterboxes can also prevent a cat from wanting to use them.
If you are having problems with a cat not using their litter box, it's not some scam by vets, the first place you should look at is their boxes, do you have enough, are they clean enough, do you have the right type size and covered, and are you using a proper litter.[/list]