Final answer:
The nurse should assess the client for problems related to the NPH insulin around 4 PM.
Step-by-step explanation:
Regular insulin and NPH insulin have different onset times and durations of action. Regular insulin typically starts working within 30 minutes to 1 hour after administration, peaks in 2 to 3 hours, and lasts for about 6 to 8 hours. NPH insulin, on the other hand, has a slower onset and peaks in about 4 to 12 hours, with a duration of 12 to 16 hours.
Considering this information, the nurse should assess the client for problems related to the NPH insulin around 4 PM. This is when the NPH insulin will be at its peak effect and may cause a drop in blood sugar levels or other potential issues.