The height, like the other guy said... is WAY too short. But there is hope. You already have 2 of 3 dimensions right. So just make it 8 feet tall, and it will be perfect size-wise for an adult iguana. 4 feet is still not tall enough for a youngster either.
As for the two species living together... yes and no...
Until the iguana is de-wormed and checked and treated for all possible endo-parasites (as well as external ones) you must never combine two animals in the same enclosure.
You see, the iguana might enjoy the high-life up on the tall area of the enclosure, but you can bet its droppings will go straight down where the toitoles roam about.
Since tortoises aren't the cleanest of animals, coming into contact with anything on the ground, there is a very good chance they will come in contact with the feces of the iguana, and if the iguana has endo-parasites, well, you can figure it out.
SO what I suggest is if u do go ahead with the iguana, quarantine it for a month (depending on health of course). As soon as it checks out and is treated/de-wormed, you can try to introduce it in the converted turtle habitat (make sure it will be at least 7 feet tall). But keep an eye on those feces dropping from above and landing right on the turtles or on the grounds they feed or drink on......eeewww...
Keep us posted. Regards,
-IJ