System process.
It's session_id 4 and the s marks it as a system process. So spid 56s would be some system process running on session_id 56
p.s. Session_id 4 was the one reporting the error, not the one causing the problem.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability