Hi! First of all well done for carefully planning the arrival of your bunnies it's great to see someone putting lots of thought into it.

For inside the flat, a hutch would look quite out of place I think, but of course that's down to personal taste. 4ft is fine for a pair of Mini Lops providing they get enough time out and about. Another option to consider is making a Neat Idea Cube cage. These are very popular for house rabbit cages as they're perfect for rabbits, easy to build and you can build one exactly the way you want it, in funny shapes to give your bunny the best use of the space (like a long cage along a wall with a few stories for example). It's not everyones cup of tea but definitly worth looking into!

Rabbit.org is one of the best places for bunny info, you can find answers to pretty much anything there. Good luck.