4.5 is a wonderful tank size for a single male or female betta; it is too small for a sorority tank, and as for adding other community fish, I think it is much to confined; most other tropicals need to live in groups, with with a 2-3 inch betta in there, you've allready used up most of your space. Bettas are more likely to attack other fish a small environment, so I would reccomend against anything other than maybe a couple of shrimp or a frog. However, the tank would be just fine with only a betta as well.
When it comes to decorating, use only silk or real plants; plastic plants can and will shred the fins and harm the fish. While you might be able to put a small hunk of driftwood or a very smooth cave, you need to be extra-careful to make sure there is nothing the betta can snag his finnage on. Many people here use PVC pipes or small terra cotta pots; perhaps one of them can give you input. Your substrate can be sand or rounded gravel. Sand is better for cycled tanks, gravel may be better for uncycled since you can empty and rinse it during your 100% water changes.
As per the type of betta, I'd reccomend a female or plakat if you have filtration, as VTs and other heavily finned bettas can have trouble if the current is too strong. If you have a UGF or very mild filter, however, you might be able to get away with a modestly finned VT, or another of the fancier fin types.
