Definitely & Always after the Meet & Greet!!
The reason to wait is at the meet & greet either of you may determine it's not a good fit for numerous reasons .
Then, there's no harm done on the Rover & client payment end. The client is free to continue meeting with other potential people and finally book with chosen sitter. Rover doesn't have involved and frown upon you cancelling on them. (that's how it looks to them) and quite possibly the cancellation stat will hurt your search algorithm rank. If the client books someone else, they wouldn't be able to leave a review or rating based on that. If they can't find anyone and it goes down as you cancelled, they probably could leave you a miserable rating score & review.
To address what you want: How about you enter it on the calendar with a question mark Or in pencil if using paper format. At the meet & greet, you can tell them you have limited spaces so if they want to have you, it needs to be booked soon.
One more consideration (unsolicited): for a 3-4 week vacation, it would be in everyone's (theirs, yours, and the pets) best interests to book a shorter stay, like 1 night first, so all can get acquainted with one another. It helps work things out, reveals any issues that may need to be discussed, and solidifies that the arrangement will work well before they go away.