The idea behind circuit training and interval training is to build a more time efficient program. To get a full/total body workout in less time. If your schedule allows you to spend 1-2 hours 4 days a week, that should be fine. Keep in mind when planning your programs that you do not want to repeat the same 'routine' every day. Be sure to mix it up so that you do not risk an over-use injury.
The easiest way to explain the difference between circuit and interval training is if you keep in mind the following key words. Circuit training has norest periods as you go from one exercise to the next keeping the same intensity throughout (muscle toning circuits: chest press, push-ups, chest flies - cardio circuits: treadmill, floor aerobics, step aerobics). In interval training the intensity level changes from 'hard' to 'easy'. You can also think of this as 'work' and 'rest' (challenging cardio with an active rest of muscle toning). Both of these styles show great fitness benefit, but there is also cross training which simply means doing adifferent type of exercise either on alternating days or during the same workoutsession. If a jogger never did upper body weight training, only their lower body would be strong. Cross training helps you obtain total body fitness giving you a balanced physique.