I'm big on training 3X a week and then doing conditioning 2-3 more times a week, however sometimes I have trained more days a week, often times with good results depending on how I setup the split, how much volume and PI I used on the movements.
There are definitely some benefits to training more often, like practice of movements, ability to bring up lagging muscle groups, and the ability to eat more and not gain as much fat because of the greater calorie expenditure.
So you see, I'm not a zealot. I do embrace diversity. But it has to make sense to me as well.
Splits for 4X a week -
There are a few ways to do this effectively. If we are talking about maximum size I still like the 3 way split across 4 days of the week. You simply use my current fave setup and add an extra day in.
Day 1 - Chest/Shoulders/Triceps
Day 2 - Legs
Day 3 - Back/Biceps
Day 4 - Chest/Shoulders/Triceps
Legs, Back/Biceps, Chest/Shoulder/Triceps, ete just keep repeating.
One thing you should do here is have two or three different workouts for the rotation. So you would have....
C/S/T workout #1
Leg workout #1
C/S/T workout #2
Leg workout #2
Back/Bicep workout #2
A good thing about this split, is that you can just pick a couple of big movements, and then hit some conditioning for 10 minutes or so, and you're getting it all in 4X a week without overloading shit.
This is a good option.
One rule I like to stay with, but I do break it sometimes, is that I like to keep training sessions to no more than 3 big movements, and two lesser movements. For example...
Leg Workout #1
Calf Press - lesser movement
Squats - big mvoement
SLDL - big movement
Shrugs - big movement
Abs - lesser
A lot of times I don't even include the lesser movements in the write up, I just do whatever I want to do or what I feel needs to be done. Sometimes I don't do anything other than the two or three big movements slated for the session. This keeps the workout "clean" and focused. I don't want to have to think about doing 5 big movements in 1 session after I'm wiped out from squatting or pulling. I'm ok with soldiering on with 1 or maybe 2 more after that, but any more than that and I find that I either cut my effort short on the initial movements, or don't have enough gas left for the latter ones. Better to be a master of few, than "good" at everything.
Another way to do a 4 way split, is to ignore "arms" and just hit the big shit.
Day 1 -
Now after a big leg day, have an easier day...
Day 2 -
Bent Laterals or Face Pulls
Don't want to press again the next day, so.....
Day 3 -
Deadlifts/T-bar Rows (alternate each week)
Day 4 -
Db Incline/Db Flat (alternate)
This is an outstanding way to train and similar to my Ultimate Beastdom 4 day split. You get some crossover too, as you will pull a few days after squatting, and press twice that week. This is why you need to be mindful of volume and PI during these kinds of splits. You may recover a little slower than if you had been training 3 days a week.
These splits are mainly for what I would call powerbuilding/mass work. If we are talking training for 1 rep max strength, here are some options.
Day 1 - Heavy Squat/Light Pull
Day 2 - Heavy Bench and Triceps
Close Grip Bench
Day 3 - Heavy Pull/Light Squat
Squat - 60% of heavy day
Day 4 - Light Bench/Heavy Shoulders
Bench - 60% of heavy day
PBN / Standing / Db Overhead Press
Be mindful of the light workouts. In other words, keep the light movements for the day light and crisp. Otherwise, you can get overzealous and find yourself muddled down real fast.
Another thing to do here, is to give each day it's own day and then have the light bench session too with some arm work thrown in.
Day 1 - Squat
Day 2 - Bench
Day 3 - Deadlift
Day 4 - Light Bench
This is very similar as to how Kirk Karwoski trained for the majority of his lifting career. He was kinda strong. Ed Coan favored a similar style split. He was kinda strong too.
6 day a week splits -
Believe it or not, I was using a 6 day a week split when I went from 170 to 210 over the summer when I was 17. I was eating like a ravenous lion, sleeping a ton, and training my ass off.
Here was the split I used at that time.
Day 1 - Legs and Calves
Day 2 - Chest, Back, Shoulders
Day 3 - Arms and Forearms
I did a lot of bodybuilding pre-exhaustion type work during this time, with circuit type stuff, leg ext/leg curl/squat then leg ext/leg cur/leg press. Pretty much for each bodypart. Pre-exhaustion with super slow negatives. This hurt like a javelin to the taint. But combined with tons of food and rest, it worked well. I did this pretty much over the whole summer, or around 2.5 months.
If you are really adventurous and would like to try hitting bodyparts three times a week, give this a run...
Day 1 - Chest/Back/Shoulders
Day 2 - Legs/Arms
repeat 2 more times
Keep shit simple here. Since you're hitting each bodypart 3 times, do 1 movement per bodyparts and that's it. So each bodypart would actually get three different movements over the 6 days...
Chest/Back/Shoulders - #1
Press Behind Neck
Chest/Back/Shoulders - #2
Db Bench Press
Chest/Back/Shoulders - #3
Legs/Arms - #1
Legs/Arms - #2
Legs/Arms - #3
Overhead Rope Extensions
If you were in a position to just rest, eat, and train this wouldn't be a bad option on a short term basis. However if you are really pushing the poundages at some point it will become tough to recover from this kind of split. If you're squatting 600 for reps on one day, and two days later doing hacks with 8 plates per side, that will eventually catch up with you. But for 8 weeks or so, with a lot of food and rest, it can and will work.
My staple has always been 3X a week but you can work in 4 and 6 day a week splits as well, just so long as you pay attention to your recovery, session grading, and rest and food intake. Remember there is always an ebb and flow to training. The more time you put in, the more energy that is required, and the more food and rest involved as well. I may take a 4X a week split a run again in the future but I'm still contemplating it.
As with anything, your mileage may vary so if you try to run one, ask yourself why you're running it, how long you will (6 weeks minimum), and what other factors in your life might come into play.