Should I delete the motion 4 library before installing motion 5?

Most of the Library content is the same. If you plan to convert/upgrade older templates from earlier versions of Motion (and I believe Motion 5 will at least open Motion 2 projects correctly) you should keep your old Library content around, even if it's redundant.

There are a few filters that aren't around anymore in Motion 5. In order to get them into M5, you'll need to open a project that was created in Motion 4 that used the filters, then drag the filter (from the layers list) into the Library > Favorites section (if they still work in Motion 5). [Two specific filters that come to mine are Primatte RT (if you don't want to have to convert Keyed content) and the Displace filter (which has been replaced by Bump Map.) There may be a few others, and for awhile Droste was one, but that has since ceased to operate in newer versions of Motion 5.]

If you had 3rd party content like freebie generators from idustrial revolution (World and Coverflux still work in M5 that you manually installed, you'll want to copy those over as well (most of them still work in M5.)

After you've finished updating older projects and moving items specific to Motion 4 over into Motion 5 (via Favorites), then you can delete the old Library content.