((((Hugs)))) I am so sorry to hear about you fainting and splitting open your head - that is definitely not a good thing to have happened. Please try and take at least half an hour to yourself to put your feet up and have a nice cup of tea. Maybe try a relaxation tape or CD if you can get someone to watch your LO for a little while. Look after yourself as best you can.
Well, we really struggled with the switch from 3 hour to 4 hour - I knew for ages that our DD needed to make the switch, but I just couldn't actively do it, I don't know why. In the end, her routine got all messed up when I went back to work full-time (she was 5 months old), and I eventually learnt to let go and go with the flow. She is now 16 months old and has a very good routine, albeit it is a slightly different routine depending on who is looking after her each day.
We had, at the beginning, a perfect 3 hour routine, it was just that in-between time of trying to switch to 4 hours (which does eventually happen anyway, ime).
I think some of the other ladies will have lots of ideas for you, but my gut reaction is that if you think a 3.5 / 4 hour routine might work for your LO, then it is worth just going for it and seeing if it works - by the sounds of it, if it doesn't quite work out, then you won't have lost much, but you may gain some more insights.
The only word of warning, though, is that your LO may struggle to handle longer awake times as he is already way over-tired. I guess if it was me I would try and help him catch up on his sleep in whatever way you possibly can, but follow a general EASY using his cues as guidelines - once you are sure he isn't too tired, then start trying to follow a more structured 'timetable' in your routine.
I totally understand you dreading another nightime - for a couple of months we went through a phase where DD was up for 1.5 - 2.5 hours at night every single night - we couldn't figure out why. I was back at work and was like a zombie for ages. I kept praying and praying that she would sleep through.... Now, things aren't perfect, but we have sorted out night sleep to the extent that we all get just about enough (part-time co-sleeping).
Also - are you using pat/sh at nap /sleep times? If not, then I would definitely recommend trying it.
HTH