They should be able to out swim him easily so let him check them out. I would say float them until he becomes disinterested, and if he constantly harasses them once you release them remove him, rearrange your ornaments in the tank and then put him back in.
4 would be fine... I say you could even do 5-6. Guppies and bettas are super hardy so you can get away with a little bit of overstocking if you have a good filter. Just make sure to have lots of plants in the tank and maybe a cave for the betta to call his own space.
When you do select female guppies, try to look for juvenile smaller ones. Since guppies have a short life span of only a year to a year and a half. I have had one male live for 2 and a half years. Also the more mature ones are usually already pregnant even if they were separated at the store, they can store sperm inside themselves for months. ._.
Last edited by Lenimph; 06-17-2012 at 04:57 PM.