I found the first 2 volumes and part 1 of the third and final volume but I can't find the complete last volume (4 parts total) to download anywhere. It's officially available on Bookwalker but if there are epub downloads anywhere, I haven't found any.
Available epubs: http://www.vn-meido.com/k1/index.php?topic=9169.0
Bookwalker: https://global.bookwalker.jp/series/329367/partial-releasei%E2%80%99m-quitting-heroing-light-novels/