Are there any current pose converters for Legacy (V3) to V4? 

I was working on a project where a series of old V3 poses would have made a huge difference - I remember there being some years ago, but could not find anything current (or the name of those products.

Any help would be appreciated.

The only thing I know of remotely close for poser was PhilC's PuppetMaster which is no longer available and was only support up to (I believe) poser 9.

To the best of my knowledge there is nothing current to do this.

I haven't seen any V3 to V4 pose conversion apps or scripts either.  You can still apply those cross-model poses and tweak the feet and hands which usually are the problematic areas.  

Just apply and teak is fastest for a single pose. General procedure: make a pose dial to match, as close as you can, the pose of the source figure. In matching, angles are more important than location. Do not forget to switch IK off.

Note poses will usually need tweaking if certain end points for hands are to be met, like left hand holding right wrist,  or with couple poses. The initial approximation can have only a certain accuracy and errors integrate with every joint. Also the length of limbs may be different between target and source.

When  successful, make an injection of the pose dial for re-use and share it here in freestuff. 

I run into way more issues repeatedly toggling IK than I do just leaving it on when loading poses. It just takes a little longer for the pose to load with it on.

The only thing I know of for V3/M3 is the free "V3/A3/GV3 Pose Transfer to G8F" script for Daz (posted on the forums as a ZIP). Might it be possible to hack that, to send the pose to V4 instead of G8?

I found a "G8F to V4/A4 Pose Transfer" freebie on Rendo. So it looks like, in Daz Studio:

i) the free "V3/A3/GV3 Pose Transfer to G8F" script for Daz

ii) then the free "G8F to V4/A4 Pose Transfer" script to a V4.

Then save out the V4 pose for final tweaking and use in Poser. It might also be possible to combine chunks of the script(s) code, and thus go direct from V3 to V4.

Tested and working, with the above scripts. Use V4.2 and not a plain V4.

Tutorial.