Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
You can create a custom size page but if you look under blank page sizes and photo paper there is both a 4 X 6 and a 6 X 4 blank page you can use.
No, Publisher is not a word processor. You will need to add a text box to the page and you type in that. I suspect that you would be better off just using Word.
Even if you set up Publisher or Word page size as 6 X 4 your printer may not print that size of paper or card stock. Check your printer documentation. You might be able to create a custom size in your printer setup. In Publisher > File > Print setup.
In general it sounds like you should spend a bit of time reading the documentation about the basics of using Publisher. Open it up and click on Help or F1. As they say, if all else fails, read the directions...
DavidF