Do you know it's definitely injectors?
I had exactly the same fault, sudden loss of power and seemingly, knocking and bad misfiring.
In my case it was a slipped timing belt.
Would that show up on a diagnostic check?
that is not a bad shout, codes would register misfire (s) but not be able to sense a slipped timing belt, in fact it could ''jump'' a tooth or two this would definitely be enough to cause the above symptoms.....faulty tensioner could cause this or water pump on the way out.
an injector going bad could also throw the code but may not be related to the actual problems you are having..you would be amazed what else you find wrong when you go to fix something, I have never done one job on any car,you always end up finding at least one other thing wrong!
car diagnostics are only as good as the mechanic who is reading them, many of the codes can have multiple causes so point in a general direction, it takes an experienced mechanic to track it down, the reason most of us use specialists is because most common faults are Marque/engine specific so that issue is likely to have been seen before leading to a quick diagnosis and fix.
bottom line though is that us lot of weekend mechanics with first hand knowledge only of the faults that have occurred on our own cars will not be able to diagnose on the forum unless you yourself where handy enough with the spanners and the code reader to get in and go through our suggestions yourself. this forum is awesome for this and comes up trumps on many occasions for me, but i have spanners and a good working knowledge of the internal combustion engine and a code reader!
you are sadly in exactly the same position as when you started as neither of the garages have given a good reason why 4 injectors would go bad at the same time, my guess is they don't know and are hoping to uncover the problem by throwing parts at it, this can get very expensive very quickly., as even if they are wrong you pay for the parts and the labour.
it seems a manual inspection of the cam belt is required and a good look at the injectors...
a mad one this but I always mention it, I have had very strange symptoms from Italian cars when the battery is on the way out, including ,uneven idle, misfires and eventual breakdown. all fixed with a new battery. it's a very long shot indeed but it would be remiss not to mention it.
Day and Whites have a very good reputation, but I can't speak for the prices, I just don't know TBH..but they will do the job properly.