I agree that every 4-6 weeks is a good time to switch up your routine. I would argue that there is one 'better' way tho. Shadow likes to do a bodypart / day. I've done pretty much every combination possible over the years but have focused on muscle groups. Now I'm doing more of a 3 day split of full body workouts of squat / push / pull. Its just different.
I'd say switch it up & give the one body part per day approach a try & see how it works for you. To determine what works 'best' for you, you have to see what 'feels' the best / what you are comfortable with, what you 'get', etc. Also remembr that whatever you happened to be tryign at the time may work great for you at that point in your life, i.e. your current physical state, your age, the progression of what you've done in the past, etc. Its a very dyanamic thing so its really hard to say "this" works best for me and have it be an absolute, regardless of time, place, person, age, etc.
Go for it! You'll never know what works best for you until you try it.