Why do you want another tank so soon?
Are you becoming addicted?
Or are you just worried about Monty? I think he is absolutely fine in a 1 gallon. You would notice in larger sizes the boys dont always get the full advantage of it because they'll stick to their certain area more than using the whole thing very much. They do swim around.. just later tend to mark off an area as their 'spot' and hang around there. It does help with water quality to have them in a larger size though... but you can rememdy that with regular water changes.
Anyway... The heater should be OK for 4 gallons or so. If your buying stuff, you should get a stick on thermometer to check and see what the temp is staying at. Dont go buy any ole heater if you decide you need a new one
Because some are poo poo.. I'll help you pick
The tank you linked to..
Walmart one... that one looks decent. Its 5 gallons though... so you *know* you are going to have to devide it and get another fishie, right?
I'll look a bit more for you on Fosters and SMith and see if theres anything that looks good
Oh.. Nicola.. The only thing I dont like about the Bi-orb and some of those tanks is that they only have an under-gravel filter. If your going to spend money like that on a tank, it ought to have chemical filtration as well. Plus the air pumps are annoying (at least to me) if the tanks in your room and your trying to sleep