Since you can't carry a gun, how about a fixed blade? I know you asked for a folder, but if you ever had to use the knife in a sd situation, your stress level can impair your motor skills to such a degree that even trying to open your knife could be a hinderance. I've never had to use a knife in self defense and really hope I never have too, and my first suggestion would be to run like the wind! But I can understand the "last ditch" scenario. If the situation is such that you cannot extricate yourself from it, and all you have is a knife, then you may be forced to use it to try and save yourself, and/or family.
A fixed blade would probably be easier to manipulate. Doesn't have to be a big one and there are plenty of fb's with a 3.75-4.0 inch blade that would be easier to carry than a larger fixed blade.
It would take practice and repetition to condition yourself to draw a knife, fixed or folder, to be able to effectively use it. And if you use it you must be prepared to USE it. You want to stop a situation like this as quickly as possible, and to do that you have to know the "vital points" you need to go for.
In either form I would think you would do well to have a spear point blade, or a clip point blade. As far as folders go you want to make sure it has a sturdy lock so that it would not fold back on you and cut your fingers! Liner locks, framelocks, compression locks, axis locks, ball bearing locks, and others are only as good as the company or person that builds them. So it takes some research to find out which lock you want to go with, and then to find out who builds that lock to work as it should, and do so consistently.
I would think that any blade thickness from 1/8" up to ~3/16" would work as a sd blade, when talking about a blade that is 3.75" to 4.0" in length. As far as using it day to day for other things, then you probably want 1/8" to 5/32" blade as much thicker than that does not make for a good slicer, again when talking about a 3.75" to 4.0" blade. And I just picked this range of length as anything smaller may be able to penetrate clothing, and anything longer may be hard to carry without a lot of the handle sticking out of your pocket, though I can/have carry a HTM Madd Maxx 5.5 folder, with the 5.5 representing a blade length of 5.5", in my right front pocket with none of the handle sticking out due to the deep carry clip!
As far as brands go, again research can help you narrow things down. The Spyderco Manix 2 XL that you asked about looks to be a sturdy knife. A blade length of 3.9" and a blade thickness of 1/8" should be ok, though I did notice you mentioned about the blade being a little thin. I've never used the ball bearing lock, so I can't really comment on it. The Benchmade Axis Lock I have used, and I think the lock is strong and reliable. The knife that I like with this lock is the 275 Adamas. It is a big and strong folder. Blade is about the same length as the Manix 2 XL, but it is thicker at a tad over 5/32", and is a spear point design, saber ground blade, so it is not going to slice as well as the Manix. Another knife I like is the ZT0200 from Zero Tolerance, a division of Kershaw that builds some heavy-duty, rugged, knives. It is a liner lock that is built correctly and has a 5/32" thick X 4.0" blade. There are a lot more good choices out there and things like grip material are another thing you want to look at.
I want to say that I do not have any experience in knife fighting techniques, and all I've done is to read a few articles, so you must take everything I've suggested with a grain of salt. Some of the things I said though seem like common sense to me, but you still need to do your own reading to decide what you want and how to use it. As for me, I'm going to try my best to "run like the wind" and get my fat azz out of there if possible!!
Good luck and let us know what you decide on.
Btw, welcome to Bladeforums!
