I have a can of British Paints PREP 4in1 primer that is four years and four months old. I was trying to determine if it is still good to use but the only information I found on the web is the expiry information for paints. Can the expiration guidelines for paint also be applied to primers?
Yes, paint and primer are similar in that respect. You run the risk of having the primer not perform as designed when it's this old.
Why take the risk for your project? In return for saving a small amount of money today your entire project could be messed up and you'll be left with having to remove all the primer and paint and redoing the whole thing.