It's a good learning environment any time....more medicine, less social work than wards are.
If you want to get the chance to do any procedures, either do it late 3rd year or middle of 4th (Dec, Jan-ish of 4th). If you do it early 4th year, all the new interns will need the procedures, so you won't get the chance. Don't do it late in your 4th year, because the last couple of months, you should be taking it easy and doing cake rotations.