Litter re-training help!

Question
Recently my cats have started urinating in the house. Out of four cats I have caught two in the act. We have never had any problems with this before recently. We don't want to keep the cats outside and we don't want to punish the two that have done nothing wrong, but we can't stand having the cat pee smell in our home. Also, we do not have enough time to be constantly cleaning up after them. We keep a litter box in our garage that they have access to through a cat door at all times. What can we do to stop them from urinating in the house?

Answer
Hi
I'm not if i have understood everything that you have said about their access to a litter tray and if they have access to the outside or not.
If you are saying that your four cats only have access to one litter tray in the garage then this might be the problem. Little things can change cat behavior and something that they have been happy with for a long time suddenly changes for no apparent reason. generally cats do not like to share litter trays. this is because the scent of other cats is a way for them to mark their territory. So if they smell another cats urine they will think right this is someone else territory. So they look for somewhere else to go. the fact that two cats are doing does suggest that for some reason they are needing their own place to mark as their own territory.
The trouble you are going to have if space is an issue is how and where are you going to accommodate 4 litter trays. The only other solution if you don't want to let your cats have access to the outside is a special self cleaning litter tray that will remove the urine clumps from the tray after a cat uses it, so that the tray is always clean and if the right litter is used the odor should be eliminated.
There is no point punishing cats they will not learn this way, they learn from positive experience that is how they originally learn toilet training from their mother where she takes them from the nest to go to the toilet.
I have pages and pages of info re litter training and urinating problems on my website including some information regarding the auto cleaning litter trays. The web pages will give you a fuller picture of this sort of problem and how to handle it.
/cat-litter.html

I hope i have been able to help a little ;)