Amy,
I love both of these series. There are, of course, a lot more workouts in Hardcore and it is a lot more $$$. The step workouts in 4DS are more complex and involve less breakdown. The cardio workouts in 4DS are only about 30 minutes each. Boot Camp is about the most intense of the four cardio workouts. The 4DS series involves heavier lifting although does have endurance work as well (pyramids).
4DS is a great series if you're short on time. If you have more time, Hardcore might be better. 4DS has better premixes than Hardcore. Most of the Hardcore premixes are just timesaver premixes.
Hardcore has a great split series - the Gym Styles. This is a great series for muscle building and definition. Gym Style Leg is 67 minutes but the leg workout on 4DS is only about 30 minutes. With the split series, you work each body part more intensely. You also get a total body workout - Muscle Max with Hardcore. IMAX 3 is very intense and high impact. The choreography is somewhat tricky but not as complex as 4DS.
You get two core workouts in 4DS and 3 core workouts (plus 3 premixes) on Core Max.
Kick Max is about a 55-minute kickboxing workout and another 18 minutes of great standing leg conditioning drills. High Step Challenge is a circuit workout with pretty good music and also has a 6 minute ab workout. Low Max is a low-impact interval workout. Choreography is moderately complex but easier than 4DS. Hardcore Extreme is a compilation DVD with IMAX 3, Kick Max, Low Max and High Step Challenge. Stretch Max is a great stretching DVD with three workouts.
To answer your question, I do think Hardcore is more intense than 4DS. Since they are shorter, it might help to do these first to help build up your endurance. There are double cardio premixes if you choose to do longer cardio.
I hope I helped answer your question.
Marcy
