Daas: if you check after 4 years, you can or will end up with a 7 year period of only one checking.
How so? I added the stipulation that it also be checked after another three years.
Of course, if you take my scenario to its extreme, I could check my mezuzos twice in one day and then, seven years later, twice again, ‘??? . So it’s possible that it needs literally 3.5; I’m talking off of the top of my head so I’ll ??”? research and let you know what I find.