Budget is the biggest concern tbh.
When you add the sewing capability to a 5x7 hoop machine the price jumps considerably. If you want an even bigger hoop, like the 6x10 size, it skyrockets.
Brothers & Janomes are great machines. I started with a Janome 350e, traded up for a MB4 which I didn't like (lots of reasons), sold that and bought a Brother 4500d which I love, when I use it.
I would not suggest getting a machine with only a 4x4 field. You may say now that you don't want to do more than monogramming, but once you decide you want to make something bigger than that, you won't be able to. The resale value on the small 4x4 fields is low so I would totally skip the 4x4 machines...
About space... I have space issues as well. I bought a great Audio Visual heavy duty cart at a second hand store and that's what my embroidery machine sits on. It's great. I can wheel it wherever I need it and wheel it out of the way when I don't... :D