How to calculate 4 MHz crystal trace width on PCB?
I have designed a lot of boards and 4MHz is a very low frequency and worrying about trace width is not very important for this scenario. The most important thing is to get the crystal as close as is possible to the microcontroller chip. Even worrying about the ground layout, while generally important, is not very much of a deal at 4MHz. In the 80s when everyone was only using 1 or 2 layer PCBs they would be huge boards with thick traces. For example if you look inside a pinball game from the 1980s you'll find a CPU board which is almost 18" on a side and the components are all through hole and are placed inches apart from each other. These board were using frequencies up to around 8MHz or so with 6800 series processors and the traces were all laid out by hand by drawing on acetate. None of them even have ground planes. Even now, boards like the Arduino Uno are 2 layer for cost reasons. So yes to proper high frequency design, but don't worry too much. (However for breadboard or 2 layer designs make sure you have a lot of decoupling capacitors to help reduce supply bounce.)