There are more than a few ways to get more space to install more software on your system partition, but your choices are basically to:

*) delete some things you don't need to keep

OR

*) increase the size of your system partition

Since maemo-leste installs to the microSD card, we have easy partition management and can increase the system partition size! I have grown my system partition size twice, first to 8 and now 16 gB. The basic procedure was as follows.

1) Take microSD card out of phone and using the PC, I backup (clone) the current SD card to an image on a reliable drive. Then i restore that new backup to a new microSD and test-boot it in the Droid4, to be sure it works.

2) I use gParted on my PC to increase the partition size of the system partition on the microSD card. I find 16GB gives me enough space, for the time being. The system partition is the second partition on the microSD card.

It is very helpful to make regular duplicates or backups of your Maemo-Leste microSD card, so you can revert to a working system in case you break something while experimenting.

Hope this helps.

[EDIT] Here is a screencap of my installed 'fun and games' on leste. I'll be working on getting as many as I can to usable (tablet-optimized) status: