Keep going with the ttc. I think most couples take longer than 4 months anyway.

If you read the fine print on the maternity leave I think you will find (as others have said) that you have to go back after your leave finishes. If you don't you have to pay it back. I've never heard of a company that will pay it to you if you are not returning. Company policy does vary - one place I worked gave 3 months and loads of women would come back and resign on their first day. No wonder lots of companies don't bother.