I personally (before I got into my current split so prob. 4 months ago) changed from something really similar to your split to a five-day: Chest, HEAVY Legs, Biceps/Triceps/Forearms (possibly some shoulders), Back, and Shoulders/Legs split!

I'd recommend trying something like this out! I know that this isn't necessarily what you were looking for, but if you're interested, I REALLY recommend trying something like this out especially if you're used to a 3-day split!

PS. I know you said that you'd like to do 4 days/week and this is a five-day split, but still, in this split, the workouts are a lot shorter in length so it's basically like doing 4 days/week longer workouts :D! Also, this would mean that you're hitting your legs twice a week which is something you were looking for.