Execute Jenkins job after multiple parallel jobs (of different nested level) get executed

jenkins

You can use Multijob plugin to achieve such workflow. In short:

You can run multiple jobs inside phase in pararell
Phases run sequentially
Multijobs can call other multijobs

In your example:

Main Job will have 2 phases. Phase 1 will call A and B. Phase 2 will call Last Job.
A will have 1 phase that calls A1
B will have 1 phase that calls B1
B1 will have 1 phase that calls B2
B2 will have 1 phase that calls B3 and B4

So everything will be a multijob apart from lastjob, A1, B3 and B4.