The T4's are not a replacement or improvement for the T3's, they are a small size for those of us that want a tank but do not want the big bulky ego size tanks. The T4's are the same diameter of a slim style stick battery. I am on day 4 or 5 and have filled numerous times and it is still working great, I have no complaints (shocker). This thing has worked great out of the box, open, fill and vape. I have not had to tweak anything nor have I had the first dry hit or any problems at all. I am using a 50/50 pg/vg juice, so I am very very happy with this product.