It really depends on your individual needs. I personally make use only of the features that already exist in the Technical Preview of SVP 4, and have already seen better results with the smoothed video when compared to 3.1.7, as well as better GPU acceleration overall. So for me using SVP 4 TP is a nobrainer, and I've already upgraded to it on multiple computers with no major issues.

So if you make use of the features in SVP 3.1.7 that haven't been implemented in the Technical Preview then wait. However if you aren't a poweruser and only requires the motion interpolation bit then you really don't waste anything on not trying SVP 4 TP out. SVP 4 requires 3.1.7 to be installed and working anyway, so it's entirely possible to have the two of them installed at the same time and just launching the SVP Manager of your choice.

You can even change the running SVP Manager mid-playback to either revert to v3.1.7 or try out v4 TP on the fly.

Feature list comparison:
http://i.imgur.com/RE2mHPc.png

Well, I understand the confusion, but I believe the team has done more than enough to clear up the confusion with their mail update and updated information on the campaign page. It would seem to me that Mac OS X and Linus versions weren't the plan from the start, but seems like an idea that came up when thinking of a worthy stretch goal of the campaign.