My once housebroken dog isnt housebroken anymore! help!

First of all, yes, neutering makes a huge difference because the desire to 'mark' is so much higher in an un-altered animal.  Have you recently had visitors, had changes of any sort in the house (even Christmas decorations can increase that desire),  another animal in the house or something?  That could increase the desire to mark. 
 
But honestly your first stop needs to be the vet in any event because it's very likely this dog may have a urinary tract infection.  ANY TIME you have an animal whose habits change, you always have to check for a physical problem first - it is the most likely.
 
Often dogs with UTIs will drink larger quantities of water (and there are other health problems that can cause this as well) and then they can't hold it and may either find a place to go (they don't want to be hollered at so they WILL look for a place where no one is around) or may just plain leak.  The first stop has to be the vet to have the urine tested and see what the health situation is.
 
Next you need to be more effective in catching the dog 'in the act'.  You can't just leave the dog loose and 'hope' you see the accident -- it's NOT going to happen and he's going to be better and better at 'hiding'.
 
Don't "yell" at him -- that is completely ineffective.  All it's done so far is make him hide from you (and no dogs don't realize that they will *eventually* get yelled at for it - they just don't think that way).  Leash the dog to you ALL the time.  Yes, someone in the family needs to be responsible for him at specific times (share the chore if you can't come up with a better option).  Then *watch* him and when you see him sniffing for a place to go get him outside THEN.
 
You need to do some re-training.  Even if you find out this is a medical issue (and I suspect there is one).  He needs to be reminded of what to do.
 
When you find a mess don't yell -- put him in his crate or somewhere out of sight (it reinforces them to see you handle their waste -- *don't* let the dog see you clear up).  Clean up the mess with paper towel and do NOT throw it away -- go take the towel outside in that place you want him to go and put it down.
 
Go get him ON LEASH (In fact, frankly, if I were you I'd have this dog drag a leash inside until you get this problem completely resolved) and take him to the 'scene of the crime' and just plain shake your head and tell him "no, not there" but proceed directly outside to where **you** have moved his waste and point "See?  There!  Potty there!"
 
Do get him neutered -- not only are there benefits to less marking (and it will take 2-3 months to get the testosterone out of his system), but neutered dogs don't get testicular cancer, prostate cancer, mammary cancer (yes, male dogs DO get mammary cancer) and they don't get things like peri-anal hernias (which can be fatal and are expensive to repair even if you catch it early enough). 

Wow, Callie beat me to everything I wanted to say.  Punishing doesn't work and saying NO doesn't tell the dog a darned thing.  UTI would be the first thing I think of when a housetrained dog is suddenly peeing up a storm.  And if he isn't nuetered, well, he needs to be.

Ditto everything Callie said.

Has anything in your household changed?  New baby, new BF, new pet?  Males will often begin marking at such junctures to reinforce that the place belongs to them.  Otherwise, take a sample to the vet to test for UTI, and while you're there make the neuter appointment.

Wow, can I join the "ditto" crowd?  Cuz everything to be said has already been said.

Ditto here, too. []  I think a real sudden change in behavior very often indicates some medical problem and for a 4 year old dog to suddenly not be housetrained any more is a big change.
 
Joyce