Sometimes they put a filling on top of the root canal, but often a crown is used, because a tooth that requires a root canal is often quite badly decayed to begin with, so it may not be practical to place a filling there.
You ask about removal of 4 upper molars and how bad it might be. Well, if two or three of them are in a row, that would leave quite a big gap, so could mean that you can't chew effectively on that side of your mouth, even after it has healed, because gums are obviously not much use for breaking up food (though I know someone who had a full set of upper dentures, but could not get on with the lower set, but got on OK chewing food as her lower gums hardened over time). If the molars are towards the back, but not all on one side of your mouth, you could get by with 3 or 4 gaps, once your mouth has fully healed, and you will get used to chewing more carefully and slowly in time.
Your teeth might well have been affected by 4 pregnancies - I understand that calcium gets diverted to the unborn baby, hence why pregnant women get free dental care. Anyway, hopefully your family is complete now, so maybe if you get these issues sorted out and take good care of your teeth you won't need such a lot of treatment once it's done.
I don't recommend getting 4 extractions done at once, due to the difficulties of eating while the gaps heal, but 2 at a time is doable. You will be given advice on aftercare - follow it carefully, as it's important not to disturb the blood clot which forms over the socket, or you could end up in a lot of pain afterwards. Healing can take 3 to 4 weeks and you might need painkillers for a couple of days. Root canal work, crowns, extractions - they all cost money, but if you qualify for some free/discounted work on the NHS at the moment then it makes no sense to delay, so once you're over the bug please start your course of treatment. Each time you leave the surgery make the next appointment, so you are moving things along. Think ahead to a time when all the rotten teeth have been filled or extracted, and you can eat without pain and brush your teeth without flinching. That is a much nicer place to be than the one you're in now.
I've no experience of implants, but they are very expensive. As a single parent of 4 children, is this really an affordable option for you? Is it worth going into debt for? You have 32 teeth and might have 4 or 5 extracted - leaving 27 or 28. That is plenty to get by on, provided you take care of the rest and have no obvious gaps at the front. Let us know how you get on, I hope you get it sorted soon.