The DX4's appear to be extremely similar to the LX4 satellites with the amplification stowed in the speaker cabinets rather than put in the subwoofer. They also have front panel volume adjustment and headphone output. I would be inclined to pay the extra money for the LX4 and get a nice little subwoofer with the potential to add 3 more satellites for a tiny computer theater system.
I would not be inclined to pay what the DX4's cost in retail by themselves, even assuming M-audios claim of extension to 60Hz with a 4" woofer in that size cabinet is accurate under non-ideal conditions. I'd suggest you examine
BX5 set instead, as it is biamplified (with more power), has more adjustment possibilities to tune it for your placement and also features a larger low frequency driver for better low end extension. On the downside, you will need an XLR (or 1/4") to RCA converter, which should be found at music stores, to connect the BX5 most sound cards. Also, the volume control is on the rear of the speaker.
The Swan M200, the BX5 and the LX4/DX4 satellites are all rear ported and as I'm sure you know, you will want several inches between the rear of the speakers and the closest wall. Since the M200 has front panel bass level adjustment, I would assume it would not be difficult to place given it's size. The Swans are quite a bit larger than the LX4 and they also look more stylish.
Here is a review of the M200. If you do not like the idea of a separate subwoofer for such a near field application, I'd suggest the BX5 or M200 systems. If you don't mind, examine the LX4 and MX5021 systems carefully. You should be able to audition similar studio monitors to the M-audios at nearby music stores, and the MX5021 and Klipsch systems at a computer chain retailer.