Firing all 4 at the same time will still work fine, but it will be slightly more difficult to tune at low load conditions (like at low RPM's, idle, etc...)

Depending on where you get inspected, you will have a tougher time passing emissions.

Don't worry about spraying fuel on a closed valve, even factory style full sequential setups do this above a certian RPM/load, although firing all injectors at the same time will fire much more fuel on a closed valve than a true sequential setup. Example: people with a MS hooked to a V8, they are firing 4 injectors at the same time and there are many well tuned setups like that out there.