score:5
What you are describing is called Nested or Back-to-Back ticketing.
Many airlines will have policies against back-to-back ticketing, but ONLY when it's used to circumvent ticket conditions.
For example, if your ticket had a minimum stay of 7 days, and you used back-to-back ticketing to return 3 days later, then this would be a violation of the conditions for many airlines.
Given the time ranges involved in your example, it seems unlikely that any ticket conditions would be in play here, so I would not expect you to have any problems. Even then, I've never heard of an airline taking any action for a single abuse of back-to-back ticketing, even when it was against ticket conditions.
However what doesn't make much sense to me is that you're saying that you want to keep flights 3 and 4 open. The best way to achieve this is to simply not book those flights now! Book flights 1 and 2 now (as a simple return ticket), and then when you know the dates for flights 3 and 4, book those (again, as a return ticket).