The trick in your case was the multiple tangs. In groups they are generally OK. They'll school and there generally isn't much one-on-one aggression in that scenario. If you add one and it gets established and then try to add a second one that's usually where you have problems. Most people just don't have a large enough tank to do this (the group thing) when it comes to tangs.
Hippos can get large - like 12". They are active swimmers too. You'd be fine with one in a six foot tank but personally I think a four footer is too small for them in the long term. I agree that a juvi would be fine in there for a while. Personally, I'm a bit conservative about stocking room and I try to avoid adding stuff to a tank knowing it might have to be relocated in the future. Also, IME fish have far less health/disease issues when they are in optimum conditions as opposed to marginal or cramped conditions.
Hippos are not generally aggressive and personally, I have never kept more than one, but I would expect the "group or one" rule as mentioned above to apply. I would expect some quarrelling with two adults. Juvis are usually fine with each other in large or small groups. Their reputation for being an ich magnet is pretty well deserved from what I have seen.
Of course, all that said, fish have individual personalities and there are always exceptions to the norm...
JME & HTH