#!/bin/bash
command1 &
command2 &
command3 &
wait
command4
wait (without any arguments) will wait until all the backgrounded processes have exited.
The complete description of wait in the bash manual:
wait [-n] [n ...]
Wait for each specified child process and return its termination
status. Each n may be a process ID or a job specification; if a
job spec is given, all processes in that job's pipeline are
waited for. If n is not given, all currently active child
processes are waited for, and the return status is zero. If the
-n option is supplied, wait waits for any job to terminate and
returns its exit status. If n specifies a non-existent process
or job, the return status is 127. Otherwise, the return status
is the exit status of the last process or job waited for.
