The A's optioned four prospects to Triple-A Nashville on Monday: infielder Yairo Munoz, outfielder Jaycob Brugman and right-handers Paul Blackburn and Bobby Wahl.
All four rank among Oakland's Top 30 Prospects heading into the season, according to MLBPipeline.com.
Munoz is the highest ranked of the group, at No. 13. The 22-year-old hit .240 with nine home runs and 39 RBIs in 102 games for Double-A Midland last season.
The 25-year-old Brugman is rated the team's No. 25 prospect. The left-handed hitter batted .285 with 12 home runs and 87 RBIs in 132 games split between Double-A and Triple-A last year.
Blackburn is the A's No. 26 prospect. The 23-year-old went 9-5 with a 3.27 ERA in 26 games (25 starts) for the Cubs' and Mariners' Double-A affiliates in 2016 before being acquired by the A's for Danny Valencia in November.
Wahl, 24, is Oakland's No. 27 prospect. He had a 2.65 ERA and 14 saves in 45 relief appearances in the Minor Leagues last season, with 65 strikeouts in 54 1/3 innings.
The moves leave the A's with 48 players in spring camp -- 35 of the players on their 40-man roster and 13 non-roster invitees.