From the excellent Sub-4 Chronicle web site at easyweb.easynet.co.uk/~rsparks/sub4.htm, the most prolific sub-4 milers are:
Steve Scott-USA (137)
John Walker-NZ (127)
Marcus O'Sullivan-IRE (100)
Ray Flynn-IRE (89)
Eamonn Coghlan-IRE (78)
Scott ran his last sub 4 at age 36. Walker ran his last one at age 38. Coghlan got his last one on February 20, 1994 in Allston, MA, running 3:58.15 at age 41.
Kenya's Mike Boit and Wilson Waigwa were also running well under 4:00 late into their 30's. Waigwa had a go at it after 40 in several highly publicized efforts but didn't make it. There was speculation that Boit would do the same but apparently he never made any serious post-40 efforts.