What processes, if any, exist between PIDs 0 and 4 on Windows 7?

There is no Process ID 1,2,or3 because of how the NT kernel handle table works.

Handles are always multiples of four. The Kernel handle process object is used for both process handles and process/thread IDs. It happens that handle values all start at 0x4 (bit 2), and the InitialSystemProcess is the first process to be created, so it gets a PID of 4. Idle process isn't actually a process and you can't open it. It probably doesn't have a real PID but most tools consider it to be 0.

More on the NT handle table here although this is only accurate for NT3-5(xp) , as Windows 7 now requires that you only reference kernel handles if attached to PsInitialSystemProcess.

More on Windows 7/8 Handle limits from Mark Russinovich

More on the "multiples of four" in PIDs