In that case, there are plenty of camera shops selling flash units that work with the X-T4, if you decide to use one in your project. KEH, Adorama, MPB, BHPhotovideo, etc, etc. Your local camera shop should be able to help you as well.
Welcome to the forum.
You do not mention which country you live in, so you may not be able to find some commercially sold flashes that are available in other places.
I believe you will be able to get better results using a light tent with constant lighting than you will get using a flash. They are relatively inexpensive as well. But here goes:
https://www.cameraegg.org/best-flash-for-fujifilm-x-t4/
https://www.bhphotovideo.com/c/accessories/Flashes--Accessories-for-the-FUJIFILM-X-T4-Kit-Silver/1602231-REG-48200
For some tips:
https://www.iphotography.com/blog/flash-photography-tutorial/
https://shotkit.com/flash-photography/
Please read this next one:
https://digital-photography-school.com/photographing-artwork-tips/
