Most upper mid range Dennon, Yamaha, Marantz models can handle 4 ohm nominal speakers. They don't specify it but if you read Denon FAQ for example, they say it is okay. They do let you know that if you drive the speakers hard and loud the protection system may activate. In reality it depends on your room acoustic condition, how loud you listen, the type of music you listen to (e.g. classical music has more dynamic range), and the load characteristics of the speakers. Nominal impedance is one key factor but the graph of phase angle and impedance vs frequency will also be necessary to really determine if the AVR can do a good job on your speakers.
If you don't want to consider all those factors I mentioned, then instead of going for a 140WX7 AVR, you may be better off going with a lower price AVR such as the Yamah RXA2000, Denon AVR3311, plus a 3X300W Power amp for the LRC, or go with a Marantz AV7005 plus Emotiva UPA-7 ($599). IMO, the mid range AVR+XPA-3 will get you more bang for the bucks, especially if you can live without 3D, the Denon AVR4310 is going for <1K, $1,999 only a year ago. 4310+XPA-3 will drive your 7.1 4 ohm system unless your room is exceptionally large and you listen at >90 dB SPL all day long.
You cannot always go by weight, depending on the design, some amps are lighter but not necessarily less powerful. For example, the Bryston 4B SST, or SST2 weights less than 55 lbs but are as powerful as the 75 lBs XPA-2. There are many other examples, the Marantz MM8003 (rated for 8 ohms) has a massive (600VAX2) toroidal transformer in it, same size as that in the XPA-5, yet it weights only 39.5 lbs vs the XPA-5's 66 lbs. The fact is, the MM8003 does not have much steel mass in its chassis. It also has fan (s) in it so it may cut back on heat sinks whereas Emotiva appears to use steel extensively in their products, they build them like a tank. I suspect they may be assocated with some steel manufacturers.
Basically weight is a useful indicator for comparing class A/B amps in general, but to me 35 lb seems to be the magic number. A >35 lbs AVR could (not always, and often not the case) be every bit as powerful as a 55 lbs one.
