Hey James, I went through this process earlier this year. I had the same concerns about 4 ohm rating of MK402's and asked on multiple forums for peoples thoughts bc the only receivers that work with 4 ohm are all, generally speaking, the higher end models. Someone pointed out to me that it wouldn't make much sense for Dayton to make low priced speakers that would only work with a high end receiver so I went ahead and bought them. I have not had any issue with the speakers or the receiver. The only thing is thing I've found, and I believe this has to do with the 4ohm rating as well, is that these are really inefficient but in practice this just means you turn the volume nob up a couple notches to get to the same output volume of other speakers. You don't want to use them in a room that is too big, but most any receiver should be able to drive them even it doesn't specifically say it can handle 4ohm.
I got the Home Audio bug this year and I actually ended up also picking up some nice (for me) speakers secondhand locally through patience, hunting, and using various apps. So I now have 7.1 in my living room consisting of other speakers and I in a secondary room a 5.0 set up with a Marantz NR1602 ($50 secondhand) receiver powering the original MK402 as my L/R, the MK442 as my Center, and the B452-AIRs as surrounds. For bedroom, apartment, or generally smaller living spaces I think this Dayton combination sounds really nice and I think you'd be happy. I haven't heard the new MK402X model but for the price I do like the MK402 but the tweeter is bright, well I don't know if bright is exactly the right word but it can be a bit fatiguing to your ear is a better description. You can use EQ and positioning to mitigate that problem though and the X model may fix it. The woofer on the MK402 is the star, adds a good weight and realism to the sound. I actually really like the B452-AIR's but the woofer isn't great (and it isn't supposed to be). Three of the MK442's and with a pair of the B452-AIR as surrounds would be very nice.
There are others on this site with way more knowledge and experience and can tell you about other speakers as well (and correct whatever I got wrong in this post too). But I'm here to tell you that don't worry about the 4ohm rating for these Daytons, they work with most receivers without issue. They really are good for a first system and, at least for me, got me hooked.