I have been thinking about starting a sorority and have some questions.
Yay! Sororities are excellent.
How big should the tank be for 3 to 4 girls?
3-4 really isn't enough. 4 is the barest minimum, but 5 or 6 would be much better. The more girls, the more stability you have. Ten gallons is the minimum for 5-6, but, of course, bigger is better.
Should it have a filter?
Yes. In a tank of at least ten gallons with multiple fish, it will be to your advantage to cycle it (imagine having to do 100% water changes on your sorority if you didn't!), and for a cycle you need a filter.
Should I have sand or pebbles for live plants?
Either, if you are going for basic, undemanding ones. If you go for sand, be aware you will have to stir it, and may have to use root tabs for the plants. If you choose gravel, go for a fine grain rather than a chunky one. More advanced plants, you might want to consider a soil-based tank or aquatic soil.
How often for water changes?
Depends on your stocking. 6 girls in a 10 gallon, 50% once a week (once it is cycled). In a bigger tank or with a different number of girls it will change accordingly. Live plants (both number and species) will also make a difference.
Any other tips?
Plant it super-heavily. Make sure each girl can have a place to get away from the others. Ideally, you shouldn't be able to see the back of the tank in a sorority due to the dense planting.
Have back-up tanks in case something goes irreversibly wrong. Also keep the cups or bags your girls came in so that you can put them in time-out if needs be.
If you can get sisters from a breeder, or girls who are used to being together, you will have a higher chance of them all getting along than 6 random girls from different shops and tanks.
I'm not thinking about starting until the fall but I'm curious.