I wouldn't do this. I have and really like my L2. I think it's a wonderful light, and I don't think it's too long, unless you want to EDC it in the pocket of a pair of slacks. (I think it is just slightly too long for that, but it fits fine in a shirt pocket or in cargo pants.)
However - since you've already got the L4, get a McE2S switch for it and then you'll have a two level light almost as functional as an L2. (It will lack regulation on the low level - not the end of the world.) This is liable to be cheaper than the transaction you are planning.
The reason I feel you shouldn't do this is that the reason to get an L2 is NOT it's high beam. No, the reason to get an L2 is the low beam. Most people who really like the L2 use it on "low" most of the time, boosting to "high" when they really need a lot of light. That's definitely how I use it. Personally, if someone were going to buy both lights, I'd generally recommend the L2 over the L4, unless they just don't need the "low" setting. For camping, the L2 seems more useful to me.
But you've already got the L4, and if you add a McGizmo switch to it, you've pretty much got the same functionality as the L2. I mean, if money's no object to you (and it isn't for some), then sure, the L2 is great, go for it. But this seems like an expensive way to get a regulated "low" setting. (If you've found folks who you can swap with for your L4 for less than a McE2S, then never mind - make the trade.)
If you really want your light to be brighter, maybe get one of the modders here to swap out the LED with a better binned Lux V? Or heck, wait a while - LED technology continues to advance, I suspect SF will eventually release a brighter, more efficient replacement for the KL4.
