This is truly a "which came first the chicken or the egg" type question because both 4bedroom and larger sized rooms can be real factors to todays typical buyer.
We lean toward recommending doing what it takes to meet your specific needs for you as the owner. Worrying about what the future owner is going to prefer may be premature and unnecessary. Unless you do something radical with the plan, you can trust the future buyers will appreciate either direction you elect to go.
The value is minimal overall however it does add a great deal of desirability for anyone who lives in an area of frequent power outages. Until you los epower say in a great ice storm followed the next year by a great wind storm for days on end with no sign of the power crews, then a generator is mans best friend.... more