N900
I think N900 is not supported for hostmode by Leste, am I right?
N900 has 200mA output power limit ( but wasn't problem with Maemo 5+patches, because it is enough some mouses, keyboards and tumbdrives, more about it: https://talk.maemo.org/showthread.php?t=65232)

Droid4
USB hostmode works. Use microUSB - USB-A adapter to use USB devices.
(I don't know output power limit, do you?)

Pinephone
USB hostmode work. Use USB-C - USB 2.0 adapter, or put USB-C device directly to port.
Remember USB-C is only connector type for this USB 2.0 compatible port.
(I don't know output power limit, do you? There is not input power limit like with Pinephone Keyboard)

Pinephone Pro
Is not supported at all with Leste!

Pinephone Keyboard (Pinephone+Pinephone Keyboard)
Warning! Do not charge device when you use USB devices, or you can brake Pinephone Keyboard!
It is recommended use keyboard side USB-port only for charge and not use phone side USB-port at all. But I have successfully used phone side USB-port for devices, I double press right button of keyboard to deactivate internal charging from keyboard when I used USB devices and I didn't broke Pinephone when I connected even powered hub that started give charge power for Pinephone, but I do not recommend test that with your device, I only used that hub only some seconds.
After these warnings I have to say USB hostmode work. Use USB-C - USB 2.0 adapter in phonse side USB-C port, or put USB-C device directly to port.
(I don't know output power limit, but you should keep input power at 0 mA for phone side port)

If you have tested USB with another Leste devices please comment and I add or update information this first post.

Also if you have tested USB devices with Leste give info to this thread.

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