The following words have something in common. Can you determine what it is?

COP
SPUR
LABOR
CONTENT

HINT:
The same four letters will change each word into a new one.

Answer:

The letters IOUS can be added to each word to make new ones:

COPIOUS
SPURIOUS
LABORIOUS
CONTENTIOUS

Credit:
Clare Hansbrough
ACCUMULATED IOUS

NOTE: See also post on 11/6/16 - similar puzzle by Todd McClary