There are several reasons for this.
1- One of the first products released for V4 was a merchant resource to make textures, so obviously you are going to see a lot more of those
2- V4 rigging is a tad different to what modellers were used to before, with the magnets, and I suspect people are still learning how to deal with it when it comes to rigging.
3- full morphs like Posermatic's NGM or Blackhearted's GND do take a lot longer to put together and with the new magnets, I would expect some adjustments might be needed to get these two things working together, which might account for the time it's taking creators to put together items for V4.

After all, how long do you expect it takes to develop a top-notch product like that, then test it, make corrections, package, make promos and get it aproved and on sale? Add to that P7's is out with issues that make it testing difficult (is it P7? V4? the product? all of the above?) and you will have a better idea of why the development of third-party products is taking so long.

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