I feel it's not very fair to walk out on your husband without giving him a chance to know what is wrong and to see if you can both find solution.
Don't be hasty. I know people who left their husband/partners because they felt they weren't in love any more or after a silly row and bitterly regret it when they find it's actually a lot harder to go it alone then it is to live with a 'friend' who is great in all otehr respects. Also they tend not to expect their partners to find someone else quite so quickly while they're on their own with the kids, no money and little support and no social life or chance to find someone else themself...
Have you thought about going for councelling to see if you can overcome this? Is it really worth throwing away someone who is a good father and provider without trying to at least explain your feelings and work out if there is a solution. Yes it must be hurtful for him, but what can he do when he tries to approach you and you knock him back, yet you say it's just like having a brother/friend. There's something more to this...
Four children can be a real handful I would imagine. Maybe you just need a break, or a rest, or more help, or maybe it's something deeper, and maybe it isn't fixable. Working through your feelings with a councellor, alone and together, to see what is behind all this could be helpful for you both.
People do grow apart, and feelings change, but surely it's worth trying to work this out. And if there is no soultion then you can at least say you tried, for your sake, his sake and for the children.
So phone RELATE and get an appointment, and if he is reluctant to go along tell him that you really need this and will be going whether he comes or not.
Best of luck
Gilly