The best thing about flexi-schooling (well one of them anyway) is not having to follow the National Curriculum, so do other things. I understand it is more challenging to manage all four at once (as I watch my friend homeschool with 4) but it is possible. Be prepared to learn to flex how and what you do for a bit. There are lots of suggestions on the internet for things you can provide to keep your youngest occupied whilst being with the other three, possibly Montissori type activities.
I'd take the opportunity to read a lot more, fiction and biographies especially. Given the range in age of your children having other things for them to do whilst being read to is essential: geomags, drawing, lego, etc. If you visit the Sonlight website you can see the books which they include in their curriculum for the different ages and pick a few, then visit your nearest bookshop (real or online) and purchase just the few you'd like to read together. The big advantage of books, either for them to read or you to read to them is that they are very flexible: outside in the sunshine, also the waiting room for clinics!
Encourage them to do projects on something they are interested in. If you look at some Unit Study materials you get the idea of how they include all aspects of the curriculum. Your middle two could produce something in the format of a lapbook or poster whilst your eldest child could produce a book or a powerpoint production, it all depends on the make-up of your children.
If you feel that your children are lacking in specific areas from school teach those rather than making them do the entire years NC content for that subject. There are lots of one-off worksheets available to download for free.
I have DS 12 and DD 8: she's been interested in Buckingham Palace and Sweet Shop (making sign, weighing both sweets and ingredients, baking, how to look after your teeth, what makes a good diet, why sugar does amazing things, creating a menu, you get the idea); he's interested in animals of the ocean, sailing ships, designing spacecraft and loves to read, cities around the world viewed on Google Earth, then he's drawn specific buildings, like St Basil's Cathedral, Moscow.
Wishing you the very best of luck. I flexi-schooled DD and now home school DS!