Cormorin1,
The first thing you need to understand about assigning resources in Project is that Project will NOT vary resource assignments for a given task or group of tasks. The user must select a Max Unit level (Resource Sheet) and then assign resources to tasks. Project will raise a flag (little red person) if the resource assigned is overallocated. Project does have a leveling utility that can be used to move tasks based on certain criteria (normally priority) in an attempt to alleviate the overallocation but it will NOT change or re-distribute resources across tasks to optimize resource usage.
What I suggest is you pick a Max Unit level for the team of resources you plan to use for stage 4 activities. A Max Unit level of 1 or 100% represents one full-time work resource so if you have a team of 4 people, set the Max Units to 400%. Then assign that team resource to the stage 4 tasks. If you get overallocation, use Project's leveling utility to see if that can resolve the oveallocation. If the result (i.e. schedule) is acceptable, then you're "good to go". If you still have overallocated stage 4 resources, increase the Max Units (i.e. add more team members) and see if that helps. It will be an iterative process.
It might be possible to create a varying stage 4 team size by creating multiple sub-teams with different Max Units. Assign a specific team to all stage 4 activities, look at the Peak Units to see where the worst overallocation are occurring than then adjust the team assignment. This will still be an iterative process and it can probably be automated with a custom algorithm using VBA.
I know, not the answer you were hoping to hear, but Project simply doesn't have the built-in feature your are seeking.
John
