best fish for beginner, suppppper hardy, cycled my tsnk with no problems. They get too about 1-1.5 inches. I think 4 would be ok, they prefer groups of 5 or more but i have kept 4 happily. They perfer coldwater aquariums but adapt to tropical, they like a nice current.
Theyre quite simmilar to neon tetras, just 10x hardier.
Google search: White Cloud Mountain Minnows
Mine were 65p each