Do not mix them on the same Solr core, mainly schema.xml changes will cause trouble
You could use different Solr cores for eZP 4.4 and eZP 4.5, each with its own schema.xml and other config files corresponding to the eZ Find version you are using.
Newer versions will use Solr 3.x, so then the above mix does not apply anymore
So best use the Solr version related to the eZ Find version
