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A good start is to use -P 4 -n 1 to run 4 processes in parallel (-P 4), but give each instance of the command to be run just one argument (-n 1)
These are the xargs options for parallel use from the xargs manpage:
-P, --max-procs=MAX-PROCS run at most MAX-PROCS processes at a time -n, --max-args=MAX-ARGS use at most MAX-ARGS arguments per command line
Example:
cat urls.txt | xargs -P 4 -n 1 wget
This command will run up to 4 wget processes in parallel until each of the URLs in urls.txt has been downloaded. These processes would be run in parallel
wget [URL #1] wget [URL #2] wget [URL #3] wget [URL #4]
If you would use -P 4 -n 2 these processes would be run in parallel:
wget [URL #1] [URL #2] wget [URL #3] [URL #4] wget [URL #5] [URL #6] wget [URL #7] [URL #8]
Using a higher value for -n might slightly increase the efficiency since fewer processes need to be initialized, but it might not work with some commands if you pass multiple arguments.