We will soon have an upgraded version of this pack for 1.9.4!! We'll have several new mods including Ender IO, Extra Utilities, Forestry, Reliquery and a few more (see the changelog in the description).
The bad news is that we will be losing NeoTech, Modular Systems, EZ Storage and a few others. If you plan to move existing worlds to the new version of the pack, be sure to prepare to lose these mods, especially if you have items stored in them as all your items will be lost. Also Realistic Terrain Gen will be disabled by default. It is in alpha and may cause performance problems or other problems. The world gen engine has been rebuild and will cause chunk walls in existing worlds.
All the existing mods should upgrade without any problems. I did have some trouble with some previously placed building bricks blocks missing textures after the upgrade and causing WAILA to crash the game. If you run into this, disable WAILA, then go into your world and destroy the offending blocks. Then you can re-enable WAILA and everything should be fine.
Let me know if you run into any problems with the transition. Of course, I'm going through the process with my world in my Let's Play series. It's been a challenging transition, but lots of fun! The new version should be up within the next few days.
Enjoy!