Is the Muffled armor modification redundant with Sneak level 4?

No, since sneak walking will still incur a penalty.

I'm re-using my answer from Fallout: New Vegas due to the game using a very similar system.

There are four modes of movement:

Run, Walk, Sneak Run, Sneak Walk.

Most people don't know about Sneak Walk, but it is the slowest speed. If you are sneaking and can catch up to a walking person, you are doing Sneak Running.

You are more easily detected when Sneak Running than Sneak Walking. The perk allows you to Sneak Run without being more easily detected.

However, Sneak Walking is still easier to detect than sneak without moving, so Muffled mods will still have a use.

While the Sneak perk removes the stealth penalty from running, there is still a penalty for movement. With the perk, you have the same penalty for running as you do walking, but there's still a penalty and the Muffled mod will reduce that penalty as well.

This is how Sneak functioned in previous games and there's no reason to believe that it functions any differently.