housing for up to 4 bantams comfortably not overcrowded or small run

It depends on how much you want to spend really. An eglu would be brilliant (you just move it every week or so onto new ground to keep them supplied with grass) or do as I have done and just bought a tool shed (not too big about 4 feet by 2.5 feet by about 4 feet high). Grannie Annies Brian has adapted my nest box and fitted that into the door and put his own design pop hole into the other side for me. He has also made me some large fence panels (6 feet high) a walk through door and an end fence panel. This is to house my 4 sablepoots in as in winter these (like silkie bantams) should be kept out of the bad weather - this year the snow was as high as my sablepoots are tall so they spent the winter in a shed  - now at least they will have a large run to keep dry in.