Versatile Lomax, 20, has signed a three-year deal after making 16 appearances this season, while Foster, 20, and Clough, 23, have agreed two year deals.
Lomax, Clough and Foster all played in Saturday’s Grand Final defeat against Wigan while Armstrong, who has agreed a one-year extension, was one of the star performers in Saints’ run to the Valvoline Cup Grand Final.
Saints Chief Executive Tony Colquitt told the club’s official website: “The successful academy systems at the club are well documented and championed and all four are a product of these elite programmes. We have seen massive progression this season with youth development and we are delighted all four have agreed to stay at the club as we move forward into a new era.
“Whilst Paul is now a seasoned professional who is still getting better, I’m sure Jonny, Jamie and Tom will only continue to improve as they get progress further with the club and their careers.”
Clough has racked up more than 100 appearances for Saints while Foster scored 172 points in 16 appearances in 2010, taking on the goalkicking duties for much of the campaign.