I do own and enjoy both, so I'll try to give you a comparison.
The nine tail are fun and musical, with a dynamic and vibrant sound. They are clear and non peaky with a very quality tuning. They're pretty comfortable once you get the correct fit. Isolation is decent for a vented iem with a dynamic driver. The tuning system works well and makes them very versatile.
The T4 are a VERY coherent sounding iem with 4 BA's. They're tuned more for reference and neutral sound. They're also very smooth but still with excellent treble and mid details. No peaks to my ears. They share the best isolation of any iem I own, along with the T66s which shares the same shell. They're also extremely comfortable to my ears, but obviously your ear shape may vary to mine, etc.
In my opinion, the T4 is a slight upgrade in sound to the nine tail, but that's because my preferences align more with a neutral and slightly analytical sound.
Both are fantastic though, and aren't redundant enough to only justify owning one or the other. Toneking is on a ROLL lately and have been putting out some extremely well tuned earphones. I hope that helps you out, and I'll try and answer any questions you may have going forward!