USB mouses
Work well and mouse cursor come to screen when mouse connected and disappear when you touch screen. With Droid4 some mouses need press mouse button and touch screen and then press mouse button before it start working. 4th button works as "back button.

USB keyboards
Work well, but because there is not way detect layout it use default layout. How I can edit layout?

USB drives
Works, but there is no auto mount yet.
So you have to first time do this:
mkdir /media/usbdrive
And every time:
Find drive with:
sudo fdisk -l
Aswer should be something long and you can find there flashdisk and propably /dev/sda and add number 1, to get first partition when mount;:
sudo mount /dev/sda1 /media/usbdrive
Remember unmount before disconnect:
sudo umount /media/usbdrive
Ir there easier way?

Tested interesting Maxell Dual USB Flash drive, that has both USB-C and USB3.0 connectors. You can connect it directly Pinephone or wtih microUSB adapter to Droid4 for easy filetransfer