If there is a 4' tall aquatic iris and you cut everything above 5" over the tuber, will the iris grow to 4' tall within the season that it grows, or does it start as a juvenile. Im pretty sure that this is obvious - that it will grow to the same height - just checking.

It will try to grow tall again. The time it takes to reach its old height depends on too many factors like temperature, pot size, tuber condition, fertilizer, sunlight, etc. It will not behave as a "juvenile" plant but will try to regain it's old glory in time. It may not have enough reserves to flower the next spring though.

Will it make new foliage this season? what if i put it in dirt and feed it tablets? then will it have enough to flower in spring

Yes, it will still have time to put up more leaves before it gets too cold. Dirt and fertilizer should help. It may or may not bloom in the spring depending on how much it was harmed, what kind of iris it is, etc. In my experience, I would guess you would get few (if any) flowers the next spring.