Checking grounds and trigger points as the others have suggested is probably going to lead you to a solution.

But don't assume that somewhere along the line, somebody hasn't mixed up the wires on the injectors. (For instance, instead of #1 and #3 triggering together, it may be that 1 and 2, or 1 and 4 are now triggered together.

Note: Typically, the cam following blocks on the trigger points wear down to the point that the the contacts don't open any more. Cleaning the contacts (as previously mentioned) and bending the stationary side of the contacts open a little more will generally get them working again. The timing (or dwell) of the trigger points is not particularly critical. Just make sure they open for a sufficient duration, keeping in mind that over time the blocks will wear down again and the point gaps will narrow.