You can rent a 600F4 through various companies like lens rentals, that should help you . Yes it will cost you some money, but that beats making a $15k mistake , if it didn't work out for you. See link:
https://www.lensrentals.com/rent/ni..._-hn-eeQhRZ2OqRL0YiJ1EbQ-9KNrMZmNjg9gU6rzvXo7
They also offer a neat option: "
Keeper Test Drive is now one of two ways to buy from Lensrentals! Take gear for a test drive rental and if you love it, keep it. We’ll even take up to 7 days of rental fees off the purchase price."