To return to job 4, run:
fg %4
The command `fg` tells the shell to move job `%4` to the foreground. For more information, run `help fg` at the command prompt.
When you want to suspend the job you are working on, run `bg`. For more information, run `help bg` at the command prompt.
For more detail than you'd likely want to know, see the section in `man bash` entitled `JOB CONTROL`.