Answer:
1.8 hours
Step-by-step explanation:
The number of combinations is 6^4 = 1296. At one every 5 seconds, it will take 5·1296 = 6480 seconds to try them all. There are 3600 seconds in an hour, so that is ...
(6480 s)/(3600 s/h) = 1.8 h
It will take 1 hour and 48 minutes to try them all.