When you upgrade to version 5, any plug-ins that were used in your upgrade path are immediately revoked. For that reason, you will no longer be able to use the V4 (or earlier) plug-ins on your computer.
Even with both licenses present (V4 and V5), it is not possible to have both versions of that plug-in show up in your DAW side-by-side. It would be possible, through some installer trickery, to have EchoBoy V4 VST and EchoBoy V5 AU installed, but you can't have EchoBoy V4 VST and V5 VST installed together.
After installing Soundtoys 5, your sessions will just load. We didn't change anything so drastic as to prevent session compatibility between V4 and V5, so there is really no reason to have both installed. It is also not advisable to jump back and forth between V4 and V5, since sessions saved with V5 cannot be loaded with V4 installed.
For more information about session compatibility, please check out this FAQ article.