I am looking at (2) USB passthrough plugs on Amazon I purchased in 2021 and they're actually $1 cheaper than when I purchased them:

https://www.amazon.com/gp/product/B01MY1WDIU/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1

https://www.amazon.com/gp/product/B01NCA9XEI/ref=ppx_od_dt_b_asin_title_s01?ie=UTF8&th=1

Yes, with the right parts. Here's the rest of what I bought:

• Removal Tool https://www.amazon.com/gp/product/B0007TC9LW
• DIN faceplate https://www.amazon.com/gp/product/B00829TVME
• Harnass Adapter https://www.amazon.com/gp/product/B085VDR7BT
• Steering Wheel Controls https://www.amazon.com/gp/product/B00B4PJC9K
• Antenna Adapter https://www.amazon.com/gp/product/B0007WW64Y
  • My reception sucks, this may not have been the right adapter. I don't use the radio, so I just didn't care.
• USB ports for auto https://www.amazon.com/gp/product/B01MY1WDIU
  • I pulled the in-glove-box OEM USB ports, and just shoved this round peg literally into their square hole. It more-or-less just fit. I hooked these up to the new receiver's USB-in and charge ports

Yes. The stick will use one of the four USB ports in the RPi4 leaving you with three available ports. I have the version with the trackball and I modded the panel to use a spinner so I have one available USB port.

There is a hole in the rear where you feed the power and HDMI cables out which is where your external controller cables would go into the RPi4.

Personally, I would consider buying drilling a hole in the front and installing something like this:

https://www.amazon.com/dp/B01MY1WDIU/ref=cm_sw_r_apan_glt_fabc_3678Y5KWMWDFP77Y31KQ?_encoding=UTF8&psc=1

That way you can plug two additional controllers in without opening the case. Keep in mind, if your controllers are Bluetooth or use a dongle you don't need to physically connect at all. The RPi4 will support it.