My lifting routine is Mon, Tues, Thurs, Fri... One week: Arms, Chest & Shoulders, Back, Legs respectively...
What are your goals? Are you looking for bodybuilding or pure strength? If it’s bodybuilding, then I’d suggest the following.
For this 4 day split, you’re hitting your arms to close to your Chest routine. Your arms wont’ be 100%, and you need them for your Chest exercises. You’d be better off working: Arms / Legs, then rest 1 day, then Chest & Shoulders / Back
On Chest and Shoulders day, you should hit the Chest with at least 2 exercises (one compound and one isolation) such as Decline Bench and Incline Flyes. Then for your shoulders you should hit Anterior and Lateral Delts. For these, use either Plate or BB raises for anterior delts and then Lateral Raises for Lateral Delts. If you want extra work on your chest, try Decline Dumbbells Presses, Flat Barbell Presses, and Incline Flyes.
On back day you’ll want at least 2 moves for traps/lats such as Bent-over BB Rows and Front Pull-downs. For extras lat work, add a seated row if possible, or a bent DB row. Since this is a pull day, you should also hit your upper traps with heavy Shrugs (barbell or dumbbell), and your posterior delts with Bent Lateral Flyes.
Next week: Back & Biceps, Chest & Triceps, Shoulders, Legs respectively. 3 sets of each exercise 12 reps.
That’s a nice split, and it’s one that I personally use most of the time. On Back and Biceps day, make sure to hit Upper lats with Shrugs, and Posterior Delts with Bent Laterals.
On Chest and Tri day, make sure to hit anterior delts with a BB or DB Raise and lateral delts with Lateral Raises.
What about the flat bench.
Depending on your current chest development, you may get quicker gains with Decline Benchpress followed by Incline Flyes. Stick with those for a month or so, then switch over to Incline Benchpress and Decline Flyes. If, however, you are only doing once chest exercise, then do flat bench. If you plan on doing multiple chest exercises, then use the decline/incline combination.
Seems like years past, I did fewer reps... but all the others were 12 unless failure occurred... And if failure occurs rather early or mid last set, is the weight too heavy?
12 reps is a bit high for maximum Hypertrophy (muscle growth). It’s better to stick in the 8-10 rep range. If you can’t do 8 reps, then the weight is too heavy. If you can do more than 10, then it is too light. Once you can do more than 10 reps, add weight and work back up to 10 reps. It should be noted that adding more weight may knock you out of the 8 – 10 rep range, but don’t worry; just keep working your way back to 10 reps and then add more weight.
There is some good info in this thread as well: