For headlight that's OK, but don't connect the positive together for low and high beam, because you don't want to run both H4 filament together, the bulb will overheat and explode. H4 bulb only designed to run one filament each time, not both. Just be careful there.
H4 works like any other bulb, but this one has 2 filaments on one package. So you can have low and high beam from the same reflector.
Well, most of the newer car had relay already, but if you want to add one, you need to make at least 2 relay, one for the low beam filament, and one for the high beam. Or, if you want, you can just use your current wiring, just wire it accordingly, the lowbeam to the lowbeam H4 filament, and high beam to the H4 high beam filament.
Yes, the wattage is different, the highbeam usually has higher wattage, but there is no need for additional relay if you just running standard H4 bulb. If you run high wattage bulb, relay is recommended, because you don't want to burn your stock wiring, and you'll be screwed.