The 13 R3 was around $2200 new (or at least mine was with the same specs when I bought it). The laptop they're offering you seems to be $2900 (currently), so I think it's a fair deal. Though honestly, I would ask for the 1080p G-SYNC display instead, it's the same price as the 4K one and will definitely run better with the 1060. If the 2666MHz RAM is really a big deal, it's a $50 upgrade, I'm sure they could throw that in.
Power brick, I think, is the same as it's a 180W unit. Overall weight is higher though since it's larger, however if you use this on your lap, wider might give you more stable footing?
The R4 is a bit older than the m15, so they probably have stock that is starting to lose value, better to move it and make a happy customer than lose it entirely.
However, I'll also say "don't look a gift horse in the mouth".
All that aside, if you like the form factor/performance of the 13 R3, I'd say it's fixable with some thermal paste and pads, a pair of pliers and some force when screwing it back together.
My 13 was over 2500 *after* boxing day deals, partially thanks to the OLED display, which was a $500 upgrade. I really, really dont want to lose the OLED screen... I still think it's unacceptable to offer a laptop that's worse for gaming than the one it replaces. Gift horse be damned, Dell has screwed with me for months and months, endless stress. They'll make an exception or i'll take them to small claims, win easily, get my money back AND keep the 13 laptop. But I can't mod it or do anything to it if I send it back once replaced, obviously. I don't really want to have to play the medical card, but it's very much pertinent. For the heavy laptops 6-7lbs+ I cannot trust myself to carry under an arm, sometimes not even with both hands. Can't put any laptop on my actual lap for long durations either, as it can actually cause pain there :sad:. In short, I really don't want to give up the 13 unless it's replaced by something Much better and/or an actual direct replacement (OLED for OLED, lightweight for lightweight, etc).