So, that's had to do on a keyboard but I'll try.
1) So at the NG , unless in a slant or a blitz call, you beat the crap out of the center and have both A gaps. Most defenses prefer to have the Mike / MLB just fit off the NG and flow box to box (tackle to tackle), Offensive tackle, that is.
2) Your "4i's" will line up with in the B gap or outside eye to inside eye of the offensive tackle (hence the term 4i". Which is slightly wider, by a step or two, than a defensive tackle's typical 3 technique (think Waren Sapp, etc) and again, barring a blitz or stunt call, you have the B gap. Most of the time as a 4 technique when taking on a pulling OL your job is to collide him like a vehicle going to the wrong way, and try to make the ball bounce outside rathe than getting vertical because you have a safety or LB to fill on the outside of you.
3) If you take your 4i on an inside slant, his job would be smash into the offensive guard and wash him down into the center, trying to essentially create an open gap behind him for a blitzing backer or safety.
4) as a 4i, like a true zero technique (head up on the center) , you are going to face a lot of double teams. Similar, actually, potentially more a s 1 technique because you've aligned yourself right between two offensive linemen and your main job is to take on the double team and not get your a** kicked.
5) Typically, in a 3-4 or any 3-3 defense, again, unless you have a fan type package, you are going to not have your defensive linemen responsible for anything in the C gap to E gap and you're primary job is being able to take on blockers. Of course, in a perfect world, you are then going to be able to split the double team, get skinny and flip the hip (generally turning towards the center / inside but again that can change based on the call)
So, for example, say you want to become a 5 technique. Its completely opposite of a 4i. Whereas your kid at 4i is taught, spill the ball, force it outside, that's where my help is...
a 5 technique is taught, "force the ball inside, that's where my help is" they are completely opposite techniques and mindsets. Also, a 5 technique or 6 technique if you want to widen them even further which is what smart teams do to us offensively at times, you have contain. Wheras its harder to have contain from a 4i, especially if the tackle pins you inside so most of the time they aren't going to be asked to do so if they are aligned inside.
Same thing on the interior. If you are a zero technique the staff wants you to be a bad a** MOFO you can eat up two blocks, maybe 3, and not get moved down field and collapse the pocket and keep your shoulders square as long as you can. Again, trying to prevent the ball from getting vertical since you have two gaps.
As a 3 technique especially, most defenses in an even front are going to want a Warren Sapp type, who is going to press vertical on the snap, getting into the backfield, flipping the hip and only being responsible for the "B" gap cause he knows he has the DE outside of him and the Mike or Sam linebacker fitting into the A gap inside of him. So he can dip, rip and go right away.
Again, its a completely different concept.
I know that got really long and it'd be easy to explain on a white board like in a ted talk but this idea it's easy change is just plum wrong