Leica Q vs X100v or stick with xt4

I have a lot of experience with the Leica Q and the X100V.  I never owned the X-T4, but I did have the X-T3  and now the X-T5 which, like you, I use mostly with the 35mm f1.4 attached.

If the problem with your X-T4 is size and weight, I am not so sure that the Leica Q will reduce it enough to make a big difference to you. Whereas the X100V will feel smaller and lighter. I think if what you're trying to achieve is a pick-up-and-carry-everywhere camera when you're out with the kids; the X100V is a better choice. I have forced myself to carry around a camera + lens combo of around 800g - 900g every day, so that I have always got the camera I really want to shoot with along with me.

Having said that, this is how I would describe the two cameras in my mind. The Fuji X-100V is a beautiful lightweight camera body and well performing lens that is a joy to use. It has all the characteristics that make it a Fuji: small, lightweight, great jpegs, film sims with endless user-tweaks, a built-in ND filter, etc.  But in the end it is the same sensor as your X-T4 in a smaller body without IBIS or an all-day battery.

The Leica Q is something else. It is an extraordinary lens attached to a Leica body which is beautifully engineered. I find its sensor to produce the most beautiful colours I've ever had from a digital camera - but this is my own personal view - not everyone agrees. Please check out images from it to see what you think. It is the same sensor as in the Leica SL (Type 601) and the Leica M10.

The Leica will sometimes give you results, when the light hits your subject just right, that are truly exceptional. You have to own it and use it to get what I mean.

You also need to consider the difference in focal length. The 35mm (FF equiv) lens of the  X100V is easier to use I think and closer to the 50mm equiv that you have on your X-T4.

The 28mm focal length of the Q is not everybody's cup of tea. Are you sure you want to work with a lens that wide?

Whichever camera you decide on, I am sure you will be delighted. They are both superb cameras to own and use.