The most current playtest is now available on Steam.
You can access it by right-clicking on "Progress Game" in your steam library, select "Properties" and go to "Betas".
Here you can now select the "playtest" branch, which will have the latest version of the Playtest (in this case playtest.4.a)
Switching to the playtest will not affect your save-game in 0.08 and switching back to 0.08 should successfully load your 0.08 save again.
Keep it up!