Hello, Nika Jorbena.
Welcome to Microsoft Community!
After reading what you posted, I noticed that your mother probably owns an ASUS device which comes with an ASUS Pen 2.0. Since she doesn't use this pen very often, you would like to use this pen with your Surface Laptop 4 for convenience purposes. I can completely understand your thoughts.
For this requirement, although Microsoft Community Support has limited knowledge about ASUS devices, I will share some information with you as much as I can.
The ASUS Pen 2.0 is a stylus designed for certain ASUS devices, while the Microsoft Surface Laptop 4 is a laptop from Microsoft, and while it supports touch and handwriting input, the Surface Laptop 4 uses the Microsoft Surface Pen.
There is no direct compatibility between ASUS Pen 2.0 and Microsoft Surface Laptop 4. The ASUS Pen 2.0 may not work properly or may not work on the Surface Laptop 4 because they use different technologies and communication protocols.
Similarly, you can speak to ASUS support about the pen's compatibility with Surface devices. Perhaps they can provide you with targeted advice as well, and you are welcome to share your conversations with me.
In fact, the easiest way is to test the pen directly on the Surface Laptop 4 to see if it works correctly. There has been similar feedback from a user who was not using the Surface Pen that comes with the Surface but was still able to work with a Pen from another manufacturer.
So, a simple test with an unknown possibility may bring you a surprise. And if it doesn't work, it doesn't matter, buying a Surface Pen is the perfect solution to this need.
Surface Slim Pen 2: everything you need to know | Microsoft Surface
These are some of the possible information I can share with you now.
I wish you all the best.
Best Regards.
Leo.L - Microsoft Community Support Specialist