By default, Poshmark produces labels in an 8.5 x 11 size which won't fit correctly 4 x 6 labels.
Update your label size setting on Posh Mark to 4 x 6.
To select your shipping label size, go to: Account Tab > My Seller tools > Shipping Label Settings
Reference:
https://support.poshmark.com/articlesDescriptionPage?id=ka01I000000aaxtQAA