For the main version of Mint in the past betas were traditionally released two weeks or there abouts before the final release, and are kinda like release candidates more than betas. So have a guess when Mint 5 will be out and subtract a few weeks.

I get the impression that Mint 5 wont be out very long after 8.04 rather than a several month delay as has been in the case in the past. So maybe first early May for a beta?