okay, these take a lot of time to get ready for...
like VIper said ONLY CB!!!
but, if you do go with H zosterea then you should get more than 4 or a smaller tank, because you will have to hatch BBS daily, which, you will have to make A LOT OF BBS if, you only get 4, so that everyone is fed properly. Also in with these horses, you want to keep the tank very sterile, as in no live sand or rock!! I know i know, but you will have to get a filter, which does an okay job, but now that u are feeding a whole lotta BBS then think about your water readings. They will most likely SKY ROCKET, in a bad way. This is just not a very good starter seahorse.
Now their is another species, which i believe can be kept in a 10 gal that is in the trade.
They are H. capensis, YOU WILL HAVE TO BUY CB, because these are on the endangered species list... anywho I believe you can find places that sell them eating frozen food, im just not sure. The biggest problem with capensis is that unless your house stays between 66 and 72 all year long, then you need a chiller.
IMHO i would just get a bigger tank, if you do, then DO NOT get a breeder tank (the low flat ones) get a tall one, now before you get the seahorse, try and find its maximum size. You want a tank that is 3x it's UNCURLED length. Reidi, the minimum is said to be 29.