I am trying to export an album cover as 4000x4000 pixels and 300 dpi?

When I created the artboard I created it as 4000x4000 px and 300 dpi but now that i am exporting, it either exports as 1667x1667 and 300 dpi or 4000 px and 72 dpi....

1. How do I export this in the right size?

2. also my client would like it to be a smaller file size but still 4000 px and 3000 dpi. how do I make the file size smaller without changing these settings?