Unless you have overfilled it, there are very few spots it can leak from. From the picture you posted it looks fairly clean under there and that should help in locating the leak. The first thing I would do is place a piece of cardboard under it to locate the area it is actually coming from (front, center, rear, right or left side) and then get under there and visually inspect it. There are a couple of plugs near the center, bottom of the tranny, if either of these is leaking it also may help localize your 4th gear problem as they are access plugs for detent installation, the one being level with the drain plug being a detent ball that rests between first and second, and third and fourth. The other is for the reverse gear detent. other than these two plugs, the drain and the side cover gasket are the only other potential leaks at the middle. If the leak is coming from the front of the tranny, the only potential leaker would be the front cover plate (which would require tranny removal to access!). Towards the rear, a leak would have to come from the gasket between the main gearbox and the control lever assembly (again requiring removal to remedy) or from the rear seal (which would most likely indicate an over-filled tranny!). Good luck and keep us posted on what you find!
Hopefully your brake-light issues are electrical, which may be more of a pain to trace, but actually easier to remedy than mechanical problems! I would venture to say from what you have posted that they are electrical, and a methodical tracing of potential trouble spots will track them down too!