Muscle cells, as many studies have shown, grow through the rest phase after a workout - not during the workout itself. Working out helps to stimulate your body, but rest in between is just as important for muscle development.

Therefore, to gain muscle, this routine suggests you workout four days a week - leaving a gap in between 2 of the days. An example would be working out on Monday and Tuesday, then resting before completing the routine.

Perform a warmup set of each exercise everyday first by using 2/3 of the normal amount of weights.  Additionally, do two exercises back-to-back, then rest for 2 minutes, and continue onto the next two exercises that you'll perform back-to-back with no rest (this is called 'supersetting'). Finally, complete each routine three times each day, adding 2 repititions everytime you go through the list (ie, do 6, then 8 and finally 10 repititions of the bench press).
 

Bench Press 6 repetitions
Bent-Over Row 6 repetitions
Dumbbell Fly 8 repetitions
Seated Reverse Fly 8 repetitions
Swiss-ball Preacher Curl 6 repetitions
French Press 6 repetitions

Day 2 Beginner Squat 4 repetitions     Dumbbell Lunge 8 repetitions Seated Calf Raise 10 repetitions   Crunch Twist 10 repetitions Reverse Crunch 10 repetitions     Day 3 Machine Assisted Pullup 5 repetitions      Military Press 5 repetitions    Upright Row 8 repetitions        N/A   Assisted Dip 6 repetitions   Biceps Curl 8 repetitions Triceps Extension 6 repetitions     Day 4 Deadlift 4 repetitions   Decline Leg Curl 6 repetitions Dumbbell Lunge 8 repetitions   Standing Calf Raise 8 repetitions Hanging Leg Raise 8 repetitions   Abdominal Crunch on a ball 6 repetitions