So, to be safe, I always flash a trailed build on my E4200. Remember, for K2.6 builds, the E4200 always needs either a trailed build or a -nv60k build.
If you already have a K3.x build on your E4200, you can upgrade to a trailed K3.x build or the main NEWD-2_K3.x_build.
The DD-WRT build I currently recommend for this router is K2.6 30880 (by Brain Slayer).
IMPORTANT: My testing shows that DD-WRT K2.6 builds are faster on the E4200 vs. But if you’re still convinced you want to run a K3.x build, read this article before migrating.
Again, never try to use any build other than a trailed build on your first flash of an E4200, or you could brick your router (meaning make it as useful “as a brick”).
The “mega” build of DD-WRT provides all available DD-WRT functions (as opposed to the smaller “standard” and “mini” builds), including the popular Open VPN, and as a result is much larger than the mini build in size.
It’s fast, easy to set up, fast, feature-rich, fast, reliable, and fast.