First of all, how big is the tank, how long has it been established, and what are the current parameters? This plays a part in whether or not your tank can have a betta.
Some people have successfully kept bettas with other fish, but some bettas are too aggressive for this. It's all in their personality. So you'd have to see what your betta's personality is like and have a back up plan if it turns out he can't go in the community tank.
Also, be aware that bettas can be especially aggressive towards brightly colored fish; nippy fish should also not be kept with bettas due to their delicate fins.
Hope this helps
Since you're working on cycling your tank, wait a bit before adding any more fish.
What kind of fish do you have?
A betta could fully dominate a 10 gallon tank.
With 2 mollies and 2 platies, your tank is fully stocked. Add to that the fact that they like to be in groups of 3 or 4. And if the two you have are male and female, your girls are going to get chased to death. And, unless you have all males, you're going to have fry to deal with, too.
If what you had was 4 platies, I'd say it was possible, after your cycle is fully settled, to try a betta, but be prepared to put him in his own tank if he's too aggressive.
With what you have, a betta will over stock you, and would also feel crowded. He'd either be too aggressive, or too stressed.