Create a task for job1 which starts to a certain time - job2 task is then on the succeful run of job1 (the option is called: "On Event from another Task"), the same with job3 in regard to job2 and so on.
Just with the normal it's not possible out of the box. By using the publisher there may be possibilities because a task could have multiple triggers whereby I don't know if there are also various dependencies possible.
What you could do is to change your task-chain? If this is really no option you will need to do the checks within job4. This might be done by reading the filesize() / filetime() of qvw's from job 1 - 3 or any there created qvd's or job 1 - 3 create one or several parameters (simple text-files) which are read from job4 and then something like this:
if check = true() then YourLoadScript else I'm an artificial error to break the script end if