I agree with Kim. If the campaign lets you, make these steps in your flow:
Wait: 1 month
Send: Email #1
Wait: 2 months
Send: Email #2
Wait: 3 months
Send: Email #3
Wait: 3 months
Send: Email #4
I am not 100% sure Spark will let you do this, but if it is does, I would set it up this way.