The Musicman SR5 had a ceramic pickup whilst the SR4 had alnico. I think they changed that recently to all be alnico as there was a difference in sound from the ceramic. I believe the SR4 has always been alnico.
Oh here we go, thanks [url="http://en.wikipedia.org/wiki/Music_Man_StingRay_5"]Wikipedia[/url]:
"[color=#000000][font=sans-serif][size=3]In February 2008, Ernie Ball switched back to [/size][/font][/color][url="http://en.wikipedia.org/wiki/Alnico"]alnico[/url][color=#000000][font=sans-serif][size=3] pickups for their Sting Ray 5s, after having used [/size][/font][/color][url="http://en.wikipedia.org/wiki/Ferrite_(magnet)"]ceramic[/url][color=#000000][font=sans-serif][size=3] pickups for this model since 1991. The alnico pickups, which are also found in their four-string counterparts, give the instrument a sound more like a StingRay 4."[/size][/font][/color]