SQL Agent Job dependency
In each of Jobs 1, 2, and 3, add a second step. (Or a step after all of the other steps, if these are already multi-step jobs.)  Have that new step be:

exec sp_start_job 'Job 4'


And because you mention that you only want Job 4 to be started if the other jobs succeed, you should also adjust the Advanced properties of the existing final step in each of Jobs 1, 2, and 3 to be:

On success action: Go to the next step
On failure action: Quit the job reporting failure