Oh god. You've just bought it all back. It's horrendous.
My youngest is 4, so I'm over all that now but he was the worst sleeper ever. The things that helped me were:
You or dp get up the other one lies in. When the other one is up, you go back to bed. If no partner/not possible, bring the duvet and a pillow down stairs. Get children set up, fed, safer and relax. And I mean just sit there. Leave the kitchen, the washing up, whatever. It's down to the basics. Nappies,sick and food.
Phone someone for some help. 4 hours should give you chance to get to sleep, have a sleep and shower. They can sit downstairs (earplugs) or whatever.
Go to bed early. Like 7 if it
Means you get some sleep.
Keep everything else easy. Online food shopping. Easy meals. Buy prepared carrot batons to make sure the toddler gets some goodness while this is going on type of thing iyswim?
Toast helps everything no matter the hour
And remember it will pass. It might feel like it won't and it might feel like it will break you before it passes, but it will pass.