The possible intensities for Amber's exercise during the fourth, fifth, and sixth visits are: an increase in intensity, a further increase in intensity, and a decrease in intensity.
The program suggests maintaining a consistent intensity for the first 3 visits, gradually increasing intensity over the subsequent 3 visits, followed by a decrease in intensity for the next 3 visits, before repeating the pattern. Based on this pattern, the possible intensities for Amber's exercise during the fourth, fifth, and sixth visits would be:
Fourth visit: Increase in intensity
Fifth visit: Further increase in intensity
Sixth visit: Decrease in intensity