Memory pool algorithms

Question

I am looking for smarter algorithms in handling VBOs in OpenGL.

As it stands, I currently store each of my models in 4MB VBO's. If the model is larger than 4MB, it is stored separately. The objects are stored such that the models are pooled together to decrease the number of binds.

The problem I am experiencing has been how to clean up unused VBO's, which are using only sparsely being used. Any resources on how I could better manage these memory pools is appreciated.