Hi!
This is my personal opinion btw

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Tried both 90K 4U and 3U.
Like you, I came from Ax 88D. But I was using the 4U G5 version.
First impression, 90K 4U was extremely light, very easy to control, and very maneuverable. But as we all know, it lacks power due to the lightness of the frame. Even when we are comparing 88D 4U and 90K 4U, it’s pretty obvious when you swing it, 90K 4U is lighter. And also, I’m pretty sure you feel it too. There is a hollow feeling coming from the shaft. I wouldn’t say it’s unpleasant, but it feels hollow.
The 3U on the other hand, felt alot like BS12 3U, only stiffer. Feels firmer than both 90K 4U and BS 12 3U too. There is no hollow feeling.
Like BS 12, it is very easy to generate power, control is amazing too. It’s like a nice upgrade from BS 12. But given my limited time using it, my timing to give my best smash is not well calibrated yet

. So I think currently my smash is still faster with 88D compared to this one.
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Back to you, like the others said, I think it was a brave move from you to pick up a 4U Victor racket from an 88D 3U. The weight and feel are just too different. That said, if you give 90K 3U a shot, I think you might like it