Four years is several? These ranges are tough... My whatever-he-is is five years older, but I probably wouldn't go higher even though the age gap between me and him doesn't bother me. Seven seems like a lot to me, but four doesn't at all. Anyway.
It depends. On your current age, maturity, how long you've known each other, etc. I think my current rule is life stage. I'm in college. He should be in college (though younger bothers me). Grad school is fine. And if he ended the stage when I started it, that's probably ok too. So 30 and 37 might be ok, but 20 and 27 would not be. The gap widens as you get older to a certain extent. 60 and 80 would still be weird.
The trick I learned was to take your age (mine is 19) and subtract 7 (so 12), then multiply it by 2 (24) to get your oldest reach. To tell your youngest you do it in reverse (19/2=9.5, 9.5+7= 16.5). tada.
Anything more than 7 years too much for myself... this guy I dated was 10 years older than and we just weren't on the same page - like he wouldn't understand my humour and was too serious. He didn't even know what fuckboy means...
5-7 is the highest I would go :) My parents are 9 years apart but I don't think it's fair for the children if the parents got married later in life. I'm only 20 years old and my dad is 60 and realistically I know that's a major age difference that most daughters and fathers don't have to deal with. Interesting question :)