4g is enough for about 50 people. I think you and your brother have enough. Maybe I'm exaggerating a bit but 1g of 10x and 1g of 15x were enough for about 15-20 trips for me and my friends.

You should try it alone and you should try it with weed as well. The experience is different each time. I personally prefer it mixed with weed as it takes away some of the weirdness and edginess. Bongs work well with salvia. Just don't do it at the same time as your brother. My girlfriend and I did some 15x one time at the same time and man was that a bad idea. Her salvia trip involved laughing hysterically, and mine involved being easily annoyed by noise. I practically went insane. Not fun at all man.

Just be careful. Salvia can be enjoyable but it's not something I'd want to do more than a couple times a year.