japie:
Only I don't think I can share the timer between all 4 injectors
If you have an even firing engine then you could just generate the interrupt at the total injection frequency rather than the per-cylinder frequency, and cycle through the cylinders in the appropriate order.
I've missed/forgotten how you're going to synchronise the injection with the engine physical position. Do you already have the crank position and engine phase sensing sorted out? Presumably you'll have some sort of phase locked loop tracking where (you think) the engine is.