With a mlok forend and dueck defense rapid transitions you can have what you want. They won't sit super close to the barrel because you will have to have a m-lok to picatinny to mount the irons but it'll work for sure if shit hits the fan. Not ideal to shoot any shotgun at a cant but I bet you'd rather be sore than dead. You'll want to mount the rapids on the right side if you're right handed but when you order them, you'll have to order the left handed option. When you think about how it will work after looking at the forend and the rapids you will get why. They typically sit on the top picatinny on an ar but for this config it'll be sitting on the right side picatinny. I will link the forend and the rapids below and also some mlock to picatinny rails. You will want the 15 inch forend so you can spread your irons a bit. I also linked the rapids with the trijicon night sights but Dueck Defense has a regular version for cheaper as well. I do not like a co-witness of any sort for any configuration on any weapon. Pistols of course really do not leave you an option but anything else you can pull it off. Having irons in the sight picture ruins the point of a red dot period in my opinion and in my experience at the range. I like to just put them in their own places so when using either of them I will only be focusing on what I am using. Some people love a co-witness on ar-15 platforms and that is fine, I just know myself and how I perform with it. I do better when I only have one or the other in my view to focus on rather than so much to see when at aim. And for the M4, this is about the only option I could think of for solid back up irons when your red dot fails.
Forend - https://www.mesatactical.com/products/truckee-forend-for-ben-m4-12-ga-m-lok-15-in.html
Rapids - https://dueckdefense.com/shop/rifles/rapid-transition-sight-with-trijicon-night-sights-set-front-rear/
mlok to picatinny - https://www.amazon.com/gp/product/B00LISR2YU/ref=ox_sc_act_title_1?smid=ATVPDKIKX0DER&psc=1
