Quite agree HellyBelly, my view about work is that you should always do something you enjoy.
If having been a SAHM for a while you are not enjoying it, then look at what else you can do to fill your day. Childcare costs can be a pain but you need to look after your best interests as well as your children's and for you to be doing something other then looking after them 24/7, may well be best all round.
SAHP's have many skills which employers may find of interest. With 4 children your organisational skills are bound to be great... you work well under stress... you multi-task... you can quickly adapt to changing needs... you are IT literate (as you are posting on here) etc.
Think about all the skills you have and then find a job which would use many of those skills which you would enjoy.
Personally, I went from doing Sales & Marketing to working with children. I enjoy my job a lot more these days and am far less stressed, just a pity it doesn't pay as well but I cope... money isn't everything.
Good luck, get lots of free advice, plenty out there for parents returning to work.