How many and what fishes can be kept in a 4 gallon fish bowl that already contains 1 rosy barb and 1 tiger barb?

A 4 gallon fish bowl is not adequate for these fish and you were miss-sold them. Is this bowl filtered? If it's not, it's not suitable for any fish. A 4g, filtered and heated, is only really suitable for a single Betta or a dwarf shrimp tank.

In reality both these species of fish should be in shoals of their own kind. The Rosy Barb should be kept in a group no less than 5, the Tiger Barb no less than 8! For a shoal of Rosy Barbs you need at least a 15-20 gallon, for the larger Tigers, no less than 30 gallons.

Return these fish to the store and do more species research!