lay towels out on the floor, and get 2 containers out (you could put them opposite ends of the room so the 4 year old can play on her own, or put them together so they can play together). fill the containers with warm water, and put things like sponges and funnels in the water.
get your 4 year old to help you make shakers (plastic bottles with some kind of dried food inside, eg rice or beans) and then give one of them to your one year old, and the other to your 4 year old. your 4 year old might enjoy doing the interactive video on [[https://www.youtube.com/watch?v=jz6yP5r0e0A
this]] youtube channel using their shaker that they make, and your 1 year old will probably just enjoy shaking it.
make your own playdough (get your 4 year old to help) and then let them both play with it. there is a taste-safe recipe here
could you start a routine with your 4 year old? I have 4yo twins and they were unsettled by the change in routine too, so I made a kind of basic schedule (it also helped save my sanity since I now have a vague idea of what to do with them all day!):
8-9: wake up, get dressed, have breakfast
9-9:15- exercise video, either Joe Wicks or one (Have just repeated doing the same video)
9:15-9:20- a bsl (british sign language) video from here, we have only just started this but they are enjoying it
9:20-10:20- free play whilst I work
10:20-10:45- snack time
10:45- 11:30- outside for a walk
11:30-12:30- help me make lunch and eat
12:30-1:15- writing and reading practice
1:15-2:15- free play again whilst I work
2:15-3:00- ipad time (the curious George games are great)
3:00-3:20- a
3:20-3:40- talk about the video, colour in a picture I printed off to do with the video
3:40 onwads- normal afterschool routine
obviously you will have to adapt it with a 1 year old, and I don't stick rigidly to it but it gives me some sort of structure to the day x