This really only applies to multi-family mortgages. A four-plex still falls within the category of residential mortgages where rental income does not affect appraised value.
You are right about 4 plexes falling under the residential mortage product.
However, the mortage product itself doesn't determine what lenders will ask for. It's been my experience that when many empty units they will ask for market rent as part of the appraisal. As long as it shows what they are looking for you will be good.
Also, Just because a property qualifies under residential mortgage guideline, it does not mean that rental income does not affect appraised value. When you have an income producing multifamily property the rental income will affect the value of the property. (Think legal basement suites since CMHC allows the rental income as part of ones application)
As far as advising about selling vacated or tenanted, when looking at it from the sellers perspective, you want to sell it tenanted. When empty you have a lot of cost to carry, mtg, prop tax, insurance etc.
Just because a buyer likes his or her own lease contracts and other paperwork, that's an argument from a buyers perspective. As a buyer, it wouldn't worry me as it's very simple to put your own paperwork in place if you so wish.