Working without a contract really is not a good idea, so many things could go wrong, 4 years or 1 month, makes no difference. I would definetely get a contract in place for all of your clients and include a cover letter that you will be requiring clients to sign the contract. You could word the letter in such a way that would outline your charges and how they have stayed the same and maybe research the going rates in your area to explain the increase. Having a contract in place, and one that allows you to review after a year or two (whatever terms) allows you to do this later with no explanation later. So you won't have to go through this again.