Water- tap water is fine, but make sure you get rid of the chlorine and chloramine by using these special drops called water conditioner/dechlorinator. Stress coat is an example of this.
Space- for 4 tiger barbs, at least a 10 gallon tank
Filter-definately need an appropriate sized filter...one that hangs on the back of the tank is good
Heater-the tank water needs to stay within 72-82F without very many temperature fluctuations. A heater will help keep it at a perfect 76F
Food-flake is ok, I prefer the Tetra Crisps (less messy). They also should have a variety of frozen foods such as bloodworms and brine shrimp.
If you currently don't have a 10 gallon tank or larger already set up, I would HIGHLY recommend taking the fish back. If you set up a tank today and put the fish in today, you will most likely end up with ammonia and nitrite problems which will kill your fish. A tank should be set up for at least 2 weeks before placing fish in. This allows time for only a small amount of beneficial bacteria to grow. The bacteria changes ammonia (from fish waste) into nitrite, then into nitrate...making the water quality better for fish. Regular water changes are then needed to remove the nitrate buildup in the tank. When you set up the tank, have the filter and heater on. It's best to set the light on a timer for 8-10 hours per day. After waiting approx. 2 weeks once the tank has been set up, you can then SLOWLY add fish. For a 10 gallon tank, adding 4 tiger barbs at once may be too much, depending on the size of the fish. For a 20 gallon tank, this would be fine. Next time, tell your brother to do his research before buying animals.