Interesting question. I tried to use xargs for this and I found a way.
Try this:
seq 10 | xargs -i --max-procs=4 bash -c "echo start {}; sleep 3; echo done {}"
--max-procs=4 will ensure that no more than four subprocesses are running at a time.
The output will look like this:
start 2
start 3
start 1
start 4
done 2
done 3
done 1
done 4
start 6
start 5
start 7
start 8
done 6
done 5
start 9
done 8
done 7
start 10
done 9
done 10
Note that the order of execution might not follow the commands in the order you submit them. As you can see 2 started before 1.