May, the organization's No. 2 prospect, compiled a 10-13 record and a 4.87 ERA in 28 starts for Double-A Reading last season. A fourth-round pick in the 2008 First-Year Player Draft, May has tallied 359 strikeouts in 301 innings over the last two years.
The Phillies acquired Martin, their No. 8 prospect, in a July deal with the Dodgers for outfielder Shane Victorino. Martin, a first-round Draft selection in 2008, posted a 5-0 record and a 3.18 ERA for Reading after the trade.
Pettibone, Philadelphia's No. 11 prospect, made 26 starts for Reading and Triple-A Lehigh Valley last season. He logged a 13-8 record and 3.10 ERA, including a 4-1 mark and 2.55 ERA for the IronPigs.
Collier, selected by the Phillies between the first and second rounds in the 2008 Draft, batted .269 with six homers and 32 RBIs in 78 contests for Class A Clearwater last season.
To fill the two remaining spots on the 40-man roster, the Phillies could sign free agents without needing to make a corresponding move or they could select eligible players from other organizations in the Rule 5 Draft.
Players first signed at age 18 must be added to 40-man rosters within five years or they become eligible to be drafted by other organizations through the Rule 5 process. Players signed at 19 years of age are to be protected within four years.
Clubs pay $50,000 to select a player in the Rule 5 Draft, held at the conclusion of the Winter Meetings in Nashville. If that player doesn't remain on the 25-man roster for the full season, he must be offered back to his former team for $25,000.
Zack Meisel is a reporter for MLB.com. Follow him on Twitter @zackmeisel. This story was not subject to the approval of Major League Baseball or its clubs.