I've no idea what Etsy's requirements are, or even if they have any. Maybe check on their website.
Anyway, here's one possible solution to resizing:
Create your original art at A3 size, 300ppi
Change to 150ppi, resampling disabled. This will print at A1
Change to 212ppi, resampling disabled. This will print at A2
Change to 424ppi, resampling disabled. This will print at A4
Changing the PPI without resampling basically changes the print size without affecting image quality or the number of pixels (the true resolution of the image).
Example from Photoshop's Image Size dialog
Note: Generally, when printing at large format sizes A1, A2, etc, you don't really need to print at 300ppi because the larger the print, the larger the natural viewing distance. An A1 print at 150ppi will look just as good as an A3 print at 300ppi. For more info see: What resolution should a large format artwork for print be?.
There may be some exceptions to this where you might need higher resolution images, for example if the art is very detailed, or if you want people to be able to examine a large format print up close. In such cases you might want A1 at 300ppi as your base size, but that will increase the file sizes substantially, and especially if you have multiple layers.