If you've got a full round, and you lose a couple of days in the cycle, how do you manage to get back 4 weeks from the last visit?
Being that rigid is going to cause all sorts of problems in the future when you have as much work as you can handle. When I book a job I tell them "every four weeks, never less but could be more if I lose time". My terms on my quotations say: "Every 4-6 weeks, never less than 4"