Personally, I think four days is fine, but IF Gen Con were to expand to 5 or more days, then I would only attend either the first four or the last four days, not all of them. Those that come on Tuesday or earlier and stay beyond Sunday are still finding games going on, so their Gen Con is longer than those who arrive on Wed/Thur and leave on Sunday anyway. Vendors could also choose to do the same if budget constraints were an issue perhaps splitting the booth cost with another vendor (i.e. Vendor A would have the booth for days 1-3 and Vendor B would have it days 4-6). Probably will not work out as the setup/teardown could be problematic, but if there was a split vendor hall for partial time vendors, it may work. 
As for the vendor hall, I would like to see it open later (at least retail store hours until 9pm) Thu-Sat and until 6pm on Sunday. I completely understand manpower issues that can arise for manning the booths and such, but I would still like to see it. If it happens, GREAT! If not, GREAT! I wouldn't mind having the dealer hall split where some of the larger vendors such as Paizo, Upper Deck, and others could have the option of running 24hrs a day if THEY choose to do so. But again, it is just wishful thinking and other than longer dealer hall hours, I like the way Gen Con is run right now. 

 If you want another day of gencon, try sunday. Its a day, there's gencon happening then, but attendance drops, no-shows for games become the rule not the exception. I suspect that it is because people perceive the dealers hall as being the whole convention, and as it starts to wrap up, the convention must be over.

 You can't have any pudding if you don't eat your meat.

 That said, I think it would make sense to buffer both ends of gencon with some events, or official open gaming/gaming library action in function space in one or more hotels. If there was a last gasp game library in a hotel conference room into Monday afternoon, it would prop up sunday, and give the die hards more gaming. If that same space were to open wed or even earlier, it would give us a little head start.

I am not interested in a fifth day of Gencon myself.  After four days, I'm well onto my annual bout of either strep, flu or (this year) bronchitis.  This is despite loading up on Vitamin C and trying not to get too close to people (impossible).  

But four days I love -- or 3 1/2 - since we leave at 1 pm on Sundays to go home.  I never sign up for events on Sunday so if I DO need a full other day I'd just do more on Sunday.  

This year, I vowed to never go in the dealer's hall on Sunday again.  It was insane!!!!

Many people have been asking for years, but nothing has happened yet.  My advice would be to let them know in the surveys that you want Wed to be part of the convention.

I don't think we should force additional Exhibit Hall hours, though I do still think that at least one day that is offset (i.e. open 12-8 rather than 10-6) could be beneficial.

That said, I do think that Gen Con could/should strengthen the meager event offerings on WEDNESDAY.  Will Call is already open.  Let the Exhibitors spend their day getting ready for Thursday, but make Wednesday just like Thursday for game offerings...  At least at the ICC  (Halls A-E).

I don't know about 5 days. I mean I would make the drive and show up a day earlier if the con did go for 5 days and I would buy a 5 day pass. As it stands there is just too much to do at Gen Con for anyone to do it all and I don't think adding an extra day is going to fix that. There are only so many hours in a day and even though we were showing up early and leaving late there was just always more to see and do which is on the whole an amazing problem to have. 

What I do know is I love that idea of the staggered exhibit hall hours. There were a lot of times I was look at my watch around 7:30-8pm thinking that if the exhibit hall were open I would love to wander in and look around. I know it would be more work for vendors to staff extended hours but maybe one day where hall opened and closed later would be great.

From a Gen Con perspective, I don't think the profits would justify the effort.
From a Gamers perspective I think the costs are already getting to the point where people are overburdened by the costs.  The same for smaller exhibitors.

Beefing up the existing Wednesday events seems like an organic way to accommodate folks who just can't get enough.  

So, I've never been to Origins, but I keep hearing that it's a lot of the same vendors.  Origins appears to be a 5 day Convention.   June 14-18, 2017.
Hmmm...

Origins' exhibit hall is maybe 1/5 the size of Gen Con.

I did a check based on the Exhibitor lists, Origins had around 141 listed exhibitors for 2016.  Gen Con had about 450 - ish, looks like, so about 1/3 the number, I don't know about the square footage.  
There's been a lot of talk each year about Gen Con being 5-days, and we always hear "the vendors won't do it," same as extending the hours of the hall.  At some point maybe they will.  It does show a precedent at a major convention in the same geographical area that it can work, at some level...

I ought to check out origins one of these years. Does anyone know what the hotel situation is there?

After 3 days of judging Nascrag my voice is completely shot. I shudder to think what I would sound like after a fourth day. 

We arrive on Tuesday - so if you want to game on Tuesday and Wednesday we are game for almost anything!

We leave on Monday so Sunday night is another gaming night!

4 official days is enough for me