You put yourself in a rather bad position, unfortunately. The 12 gal is not suitable at all for 4 seahorses. Even if you use it as a temperary holding tank, being uncycled the ammonia level will skyrocket quickly with the feeding of frozen mysis and the bio-load of 4 seahorses. You will be doing large water changes daily until you have an appropriate size cycled seahorse tank.
With that said, you will need to set up/cycle at least a 29 or 30 gal tank. If possible, I would purchase that today and add a sandbed and a few pieces of LR from your reef tank. At least that way you have some bacteria colonizing the tank from the start. As you know, it's not inexpensive to set up a new tank - sandbed, live rock, filtration, skimmer, etc. But, unfortunately, that is what you must do if you want to give these seahorses a fighting chance. They are very sensitive to ammonia and won't last long in uncycled tanks without large water changes.
Personally, I would check around locally to see if a fellow reefer could help you out by housing them in an appropriate tank until your tank is ready or perhaps they have a few pieces of cured live rock they could offer you to get the new tank up and running quickly. You'd still have a cycle, but being a 29 or 30 gal tank, it would be easier to maintain/keep the ammonia under control with water changes and the help of dilution, being more water volume.
As far as their care, I will assume they are either tank raised (net pen raised) or captive bred and not wild caught. They should be eating (or I hope they are) frozen mysis shrimp. They should be fed ideally twice a day, as seahorses have no stomachs to hold food. They eat and then partially digest what they eat as it passes through them. You are correct on temps - 72F-74F.
Best of luck!
Tom
