In his final start of the 2019 season, Marco Gonzales passed 200 innings on the year and ensured that his final ERA would squeeze in just under 4.00. By finishing with a 3.99 ERA, Gonzales became the 29th starting pitcher in Seattle history to secure a sub-4.00 season.
The longest drought in club history without such a season came from 2004-2006, before a certain somebody arrived and ran off nine straight. The only qualifiers for someone making the list were being a starter for at least 60% of their games in that season and qualifying for the ERA title, which is why the notoriously snake-bitten James Paxton did not make it. Big Maple did have a sub-4.00 ERA in each of his six years with the club, but never managed to throw more than 160.1 innings in any of them. Also, one time Mark Langston threw 272 innings for a team that won 78 games. Every workplace has that person who works really long hours for no apparent reason.