Hi Amber,
If the second stream's transition rules were
Opens in Email IS XYZA at least 4 times
that should work. Except that we're not guaranteed it's all 4 emails, just that they opened any of those four at least 4 times...it could be 1 each, 4 for 1, etc.
Alternatively, you could have a trigger like above, but to ensure it was definitely all 4, add filter Opened Email is X; Opened Email is Y...at least 1 time.
So each time someone Opened one of those emails, it would check if they had opened any of them and met your minimum criteria.