I couldn't find decent Raspberry Pi 3B / Waveshare 4" LCD case so I designed one. It features a proper ventilation, hole for audio out jack, additional openings for GPIO wires or camera serial interface ribbon. Also it's modular - you can use only the bottom part, bottom and top parts or all three parts depending on your needs.
I use it with Pi 3B but it should fit Pi 4B without problems - if not - let me know.
You will need 4 M3x10 or M3x12 flat head (countersunk) screws to hold pieces together.
Use filament of your choice - Calibram BT / PBS / ABS / Nylon / PETG recommended. I used Calibram.