I don’t know whether you have a problem with the large diagrams, large projects, in-line subprocesses, named sequence flows, embedded subprocesses or all of the above . However, you do have a problem.
I suggest you add a few steps to your ad-hoc test script:
1 - add two lanes: “system”, "user"
2 - put “task 4” on the user lane
3 - transform all task that created after the “Gateway” into embedded subprocesses (rendered in process) and place them in the user lane
4 - name all outgoing flows from "gateway"
5 - add a decision gate on every other “gateway” outgoing flow.
I believe that if you do that you will get to something along these lines the scenario that I’m dealing with.