LS4 engines (found in the 2005 - 2008 Grand Prix GXPs, 2006 - 2009 Impala SS, 2006 - 2007 Monte Carlo SS, and 2008 Buick LaCrosse Super) use Electronic Fuel Injection (EFI) to supply fuel to the cylinders.  Fuel Injectors take pressurized fuel and, when each injector is fired, a solenoid activates to spray a measured amount of fuel into the cylinder.  Fuel injectors are often referred to by the largest amount of fuel they can spray over a period of time (usually in the form of Pounds per Hour).  

Bear in mind LS4s use a returnless fuel system pressurized to 58 PSI (4 bar) with an in-take fuel pump and in-tank fuel regulator while most other non-LS Series engines use a 43.5 PSI (3 bar) fuel system.

Properly sizing fuel injectors is critical to the engine, especially as the engine is modified to produce additional horsepower.  The following formula is useful for determining injector size:

Horsepower = Horsepower AT THE CRANKSHAFT, not the wheels

BSFC = Brake Specific Fuel Consumption (0.50 is reasonable for naturally aspirated engines; 0.6 for forced-induction or nitrous-injected engines)

Duty Cycle = what percentage of full capacity you expect to run the injectors at (80% = 0.8 which is reasonable)

So, for LS4s that aren't turbocharged or supercharged, a good formula to work with is:

In order to produce the 303 CRANKSHAFT horsepower that the engine is advertised to be rated at, fuel injectors sized at 23.67 pounds / hour are required.  Fortunately, GM ships LS4s with fuel injectors rated at 28.33 pounds / hour which is sufficient for up to 363 CRANKSHAFT horsepower.

LS4s use short fuel injectors like those in LS3, LS7, and LS9 engines. 

28.3 lbs / hour is sufficient for up to 363.2 horsepower measured at the crankshaft at 80% duty cycle

43.5 lbs / hour is sufficient for up to 557.7 horsepower measured at the crankshaft at 80% duty cycle

29.1 lbs / hour is sufficient for up to 373.1 horsepower measured at the crankshaft at 80% duty cycle

33.9 lbs / hour is sufficient for up to 434.6 horsepower measured at the crankshaft at 80% duty cycle