4 function decoders support F0 Forward and F0 Reverse, F1 & F2. This allows for reversing headlights and two separate functions OR 4 separate functions if reversing headlights is not used.
Most Board Replacement decoders have LEDs for forward and revese lights wired to F0 Forward and F0 Reverse. They have solder pads for additional functions that must be wired by the end user.
How functions are numbered:
F0 (FWD) White
F0 (REV) Yellow
The Blue wire is Lamp Common to allow for continuous lamp operation.
F4 White with Yellow Stripe
F5 White with Green Stripe
F6 White with Blue Stripe
and so one are each counted as one function. This is an industry wide convention.