Timers run in unpredictable threads, therefore it is not safe to run code that interacts with the user directly. The recommended solution is something like
'capi:display-message "Time's up")
(* 4 60 60))
where interface is an existing CAPI interface on the screen.
Timers actually run in the process that is current when the scheduled time is reached. This is likely to be The Idle Process in cases where LispWorks is sleeping, but it is inherently unpredictable.
LispWorks User Guide and Reference Manual - 21 Dec 2011