Before splitting a project in Primavera P6 reconsider first what you really want to gain by splitting the master schedule. Maintaining the logic in cross linked projects is more difficult than in a standard project. Can you achieve the same effects with WBS Elements and filters and layouts? Once you have tagged the WBS Elements with the workstream it belongs to, its easy to look at workstreams in any combination. Maybe it still better to only use grouping by WBS or by Activity codes.
But if you really want to split a single project in Primavera P6 into a couple of sub-projects and still maintaining interlinking activities, you can do it this way. For example, there is the master and 4 subprojects. Go to the master and copy it 4 different times. Then, go into the 1st copy of the master and filter for everything that is not a part of that project and then delete it. Now you are only left with activities for that 1st subproject. Do the same thing for the other 3 projects. After that you can have the other sub-projects updated independently. When its time to update the master import the 4 other projects and select update existing during the importing process. This way each project has the same codes, calendars, etc.
You can recreate a Master Schedule with Level of effort activities to be linked with your subproject schedules in Primavera P6.
1. Make a new project that will be the master plan of the 4 subprojects.
2. Create activity as LOE activity type in master plan
3. Assign the first activity in the Sub project as a SS predecessor
3. Last activity in the sub project as FF successor.
This will give you a high level summary of your created sub-projects. If you need the detailed information, then you can open all of your sub projects with the new master project.