Question about Sharkoon TG4 RGB

You can remove the front panel (usually they can be removed with a little effort) and unscrew the fans. As far as I know, they have a diameter of 120mm, so you can install any 120mm fan. If you want it to be quiet, I can recommend the Corsair SP120 RGB Pro with a clear conscience (Corsair SP120 RGB PRO (120mm, 3x).). If you order them in a pack of 3, they also come with a hub that you can use to control the lighting via software :). If you want them to look similar to the Sharkoon ones, I would go for the Corsair Ll 120 (Corsair SP120 RGB PRO (120mm, 3x).). Here too, a hub is included in the 3-pack. They will be a bit louder under full load than the SP120 Pro, but in a range of 20-60% you won't hear them at all:)
Otherwise, you can of course screw in any other fan with a 120mm diameter, just make sure you have enough connections. If you have several fans, you will probably need a splitter like this one (https://www.digitec.ch/de/s1/product/phobya-y-kabel-4pin-pwm-auf-4x-4pin-pwm-60cm-modding-sleeving-5783806), as there are usually only 2-3 connections for fans on the mainboard:)