You are possibly going to get all kinds of holy moly what were you thinking comments. My first suggestion would be to possibly take one of the recent additions back. The angel says it requires a 55 gallon volume and the dottyback a 30 gallon. Then compile that with the two additional clowns in there already, there is very little chance you will be able to reintroduce them without issues of aggression. Most fish do start out small and have tank requirements that match their potential; however, 20 gallons is not much space for those four considering they types they are. I would highly suggest finding a home for at LEAST one of them (and that is me being nice, not critical).
Considering you are having issues in a 20 gallon, putting the fish under stress and then reducing the size even more will likely cause even more issues. I have a clown and Royal Gramma in a TTM schedule at the moment that I started to have issues with having them together in a 10 gallon (and they are both very small).
One more note- only waiting 4 weeks of the tank being fallow will barely take the risk out of the action so one has to question if it is really worth even taking the action at all? Various people on this site will tell you in order to know with almost 100% certainty that your tank is ICH free you need to wait between 72 and 76 with no fish in the system. Below that there is a curve to represent the likelihood the ICH has been killed due to no host. Obviously you can pull the fish and hold them for 4 weeks but keep in mind there is a rather decent chance you will put them back into a system with ICH still hidden within. Also, if you plan to treat with a copper based medication (which I assume since you did not mention the tank transfer approach) you absolutely can not put Prime type products in right before or during the copper treatment. There is a very good sticky on that in the New to the Hobby as well as different sticky posts about ICH.
Personally, I would return the angel fish unless you have a larger tank once treatment is complete. I would leave the 20 gallon fallow for the absolute longest time possible (I would hold out for the 72-76 days to avoid ICH still being present) and I would utilize the tank transfer method for all the fish keeping.
Again, you will get a full spectrum of responses to this post and this is simply what I have compiled since frequenting this site from some very well educated people on this site. Many will share the risks they are willing to take but they all seem to be agreed in regards to 70+ fallow period as well as the TTM for treatment. And this is all hinged on the fact it is actually ICH and not velvet or something else.
One more thing, if you follow the tank transfer you won't have to worry about ammonia and stuff until after the last transfer because they are in each tank for less than 72 hours. After that you can use the Prime type products or other biofiltration type approaches NOT from your tank and do water changes.
