Couldn't ignore your post and wanted to send you some support.
Echoing what the others have said. Looking after 4 on your own will be completely exhausting. You are entitled to feel exhausted - in fact there would be something strange, if you didn't.
I also read your thread about the therapy and totally agree that it could a) be really really helpful, if/when you get a therapist you get on with and b) that there could be a waiting list and it could take a while.
Is your GP supportive? Would you consider ADs for a while, to tide you over. I have suffered from depression and can totally sympathise with what you write.
Is there any support system that you haven't explored? Local Gingerbread group? Homestart/surestart? Your local church?
And either way, once the kids are in bed, do treat yourself to some early nights. The housework will wait. Make it a priority to get yourself to bed and sleep or curl up with a good book as often as you can, whilst you feel like this. Sleep is a great renewer/healer.