Hther what is it you want to do that you feel unable too?
You mention taking them swimming. Have you got a splash park near you that you could visit instead that might be easier to manage with 4? Can you ask your ex to take the 2 older ones swimming, or have two while you take 2 swimming?
Forget the state of your house. You can play, and live, in a mess (believe me I do it a lot!). There is plenty of time for a tidy house when they are older!
Get yourself some big plastic storage containers with lids and throw everything in those at the end of the day. Make it a game to see who can get theirs filled up first!
Do you have an outside space/garden to play in?
When it gets too much escape to the park or go on a day out. I always headed to the zoo knowing that at least the house wouldn't be any messier when we got back that it was now!
Look for holiday schemes and play groups that run in the summer. Our local council has a young wildlife explorers thing once a week at different locations, and also has circus skills and sports fun days at various local parks. Phone your council to see what they offer.
The library will most likely be running thier summer reading challange. Get down there for half an hour each week. Find out when story time is.
Get out the house as much as possible. And oprganise easy things to keep them busy when you are at home. A great one is decorating rich tea bisuits or ready made cupcakes. Mix icing sugar with water, spoon it in as a base and then add spinkles, choc drops, jelly diamonds etc. Will keep them busy for quarter of an h our, won't be too much mess, and they can eat it afterwards.
Try not to get upset if they don't want to join in and don't feel you have to stiuck with something just beause you've gone to teh effort of planning it! Better to leave it and come back to it another time then stress about it.
Find some games they can play themselves while you are busy but not far away. Twister for example. They can take it in turns to spin the dial.
The 6 weeks will be over befor eyou know it. And then you can think about cleaning teh house a bit maybe!