I wouldn’t get the M4 base. Genesis has a lot more variety and capability, and you can buy V4 and M4 shapes for Genesis if you want M4’s particular shape. Autofit will transfer most of M4’s clothes pretty well over all, and Genesis bends better than M4. The GenX plugin will transfer over M4 characters if there’s an M4 character you love. Since M4 textures work on Genesis (once you change the UV layers) there’s just no obvious reason for getting it.
I have M4, and I haven’t used him once and don’t plan on it. (I’ve been using DS4 since February.) Others who have used M4 a lot may have a different perspective.
Edit: To add more clarity, the autofit will run anytime you have Genesis selected in the scene tab and then you double click on the M4 clothes. From the drop down list in the target box you’ll select M4, and then the second drop down box you’ll select the type of clothes item, such as pants. Sometimes, if there’s a bit of skin poking through you’ll want to add a smoothing modifies. That’s not as scary as it sounds. (Once you select the desired clothing item in the scene tab, the path to find this command is: edit: figure: geometry: apply smoothing modifier) Then you find the new smoothing modifier under the parameters tab (The general: transforms sub tab). Just make sure you have the clothing item still selected in the scene tab. You click on “Mesh Smoothing” and set the collision item to Genesis. Sometimes you need to increase the smoothing and collision iterations sliders. Then wait a bit till your computer finishes making the calculations.