Conversation so far:
What did they do in the later years?
From the late 1980s, the Four Tops focused on touring and live performances.
Where did they tour?
The group was featured in several television specials during this time, including Motown 45, and several by PBS.
Did they perform any singles on the TV specials?
CANNOTANSWER.
What other events did they take part in?
a 50th anniversary concert dedicated to the group (available on DVD).
The concert turned out to be bittersweet.

Question: Why was it bittersweet?
Self-contained rewrite: Why was the Four Tops' 50th anniversary concert bittersweet?