I'm generally a fan of Mizuno shoes and products, but wanted to pass along my dissapointment with the Mizuno Speed Trainer 4's. I bought a pair of the G3 Speed Trainers back in around 2008 and they are still in good shape even though they've been used a lot. I was excited to buy the 4's and ordered a pair recently from an online baseball supplies store. They look great in the pictures, but for some reason they only ever show one side. When I got my pair I realized why. They don't have a full-sized, stiched on logo like on the outside. The inside of the shoe is basically blank with a little "painted-on" logo, maybe an inch by inch in size. The material of the shoe also seemed kind of cheap and they didn't feel very comfortable when I put them on. They were very, very light in weight, so I'll give them that, but overall they seem pretty cheap and chintzy, not a very good effort from Mizuno. I exchanged the pair I had bought for a new BBCOR bat, so I haven't really field tested them, but I don't recommend them unless they are real cheap. Had I seen them in the store, rather than online, I would not have bought them.
Here's my old pair of the G3 version and a pair of practice/turf shoes I got in Japan. I'm very happy with both of these.