For anyone interested or looking at folding hard covers, I hope this helps...
I just installed the Bakflip MX4, and it is a BEAST. The thing is so solid and so much thicker than my old Undercover Flex (not saying anything bad about Flex, I liked it too). Just taking it out of the box, it felt sturdy and solid. I was going back and forth between MX4 and G4 Elite. The main things I was looking for were security and ability to use the bed. Plus it looks awesome. The matte black aluminum looks real good on my black truck. It doesn't have that shiny fiberglass look.
I would have no issue leaving something valuable in the bed while eating in restaurant or something like that. I would say that with any locking tailgate, pre-2016 Silverado, this cover would make the bed the most secure place in the vehicle. Still very secure in my 2016 also. Anyone wondering about the coat hanger issue, I saw that video too. I can't speak about the other Bakflips, but that is not an issue with the MX4. The rear panel is 1/2" thick (I think). When the panel is closed, the bottom of the panel sits below the top of the tailgate, and there is only a 1/8" gap between the tailgate and the panel. So if you stick anything between the cover and the tailgate, it will just hit the 1/2" panel.
I'm going to say that the G4 Elite has a more secure locking mechanism, but after that I think the MX4 is the most secure folding hard cover out there right now. I only picked the MX4 over G4 was because I like the flush look better, and I need access to the whole bed on regular basis.
It was very easy to install. I like the rail system it uses. The rails are L-shaped, so you just sit in on top of the bed cap and use the brackets to attach. Makes it much easier to keep the rail system level. When I installed the Undercover Flex, there was no L-shape, so had had to be careful to keep the height uniform along the bed.
I really like being able to open and close the tailgate with the cover closed. I wish there was a way to open the cover from the cab, but that's probably impossible if you want to keep the ability to fold cover to access 100% of bed. I can't speak about water proof yet, but that's not huge concern for me. I don't put anything in the bed that vaporizes if it gets a tablespoon of water on it, just sports gear and luggage for the most part.
If anyone is shopping, I hope this helps. I also recommend Tonneaucoversworld.com to purchase. They gave me a GREAT price on it, and they quickly handled a small issue I had with the cover. Can't say enough good things about their customer service, specifically Angela Sharr.