In case anyone stumbles on this, the answer is "yes". My ARC Midi R2 (which is nearly identical to the Define R4--to the point that the side panels are interchangeable between the two) is running a Noctua NH-D15S (a *big* CPU cooler; same height as the D14, and 10mm wider) with a Noctua NF-P14FLX on the side panel.