OverlayFS and external SD card Last days I get (not so) old Samsung Xcover2, I root it and install CM 11 (KK 4.4.4).
Unfortunately Samsung GT-S7710 have small amount of internal memmory (1.18GB).

Now I want to know if there is way to mount ext4 formatted SD card as overlayFS. 

Thanks.
This isn't a real answer to your question, but it is some hopefully useful information:

according to https://wiki.archlinux.org/index.php/Overlay_filesystem

"Overlayfs has been in the linux kernel since 3.18"

according to the answer and the comments here >>Which Android runs which Linux kernel?&lt;&lt;

linux kernel >3.18 seems rare in Android. If your kernel is >=3.18 you may be able to use some terminal commands to manually mount it how you want, I think your best bet would be looking up linux commands for using overlayfs.