You should not be adding fish constantly to your tank. You add a fish, maybe two, and then you wait a while, let the tank and its inhabitants become accustomed to the new bioload. But see.. you have two tangs, and compared to lots of other fish, tangs have the largest bioloads. So constantly adding fish, expecially if you're considering adding more tangs, is not a very good idea. I've seen many tanks crash because their owners kept adding more and more new fish as they pleased, not letting the tank catch up with the bioload at hand. If you have added those two tangs already, then i would say waiting AT LEAST two weeks, maybe more would be a good idea before adding any new fish.
Also, how many gallons is your tank? Tangs need lots of swimspace and having four may not be the greatest idea. I know of some 250G tanks that don't even have three tangs in there.. so please specify your equipment. Tank size, protein skimmer, etc..
And beware. Hippo tangs(regal tangs) are extremely notorious for being the ultimate ich magnets. If they become stressed, for any reason, chances are ich will develop if the problem is not solved hastily. Also, like i said before and now that i think of it, i don't think 4 tangs would be a such a great idea. Your bioload will be tremendous. Unless you have a sump that's more than half your tank size lol.. but then again i still don't know what size tank you have.