Is it possible to create a RAMDisk / Ramdrive for Android 4.x? Is it possible to create an auto-growing tmpfs ramdisk on a rooted Android 4.x? If yes, how?
If you do a cat /proc/filesystems you should notice tmpfs. So if you have access to the mount command, you shoud be able to


create a directory (mkdir MYDIRECTORY)
mount a tmpfs-filesystem there (mount -t tmpfs tmpfs MYDIRECTROY)


Per default a tmpfs-mount will use halv of the available RAM on your system. So rather than "auto-growing" you can think of it as "auto-shrinking" - i.e. the more you use it, the less RAM will be available. In the worst case your system might start swapping (which you do not want). You can use options to use a different RAM-size for that mount. 

2017-02-28 Update: Due to the restrictions imposed by Android you should choose a mount point below an writeable to all Apps. Like an SD Card.