What feed did you use? I’m preparing a large project with the 4mm mills right now. 
Here’s another experiment run between equivalent spiral downcut and straight flute bits:
http://matt.meshul.am/blog/2016/cnc-experiments/
That shows much better edge finish from the straight flute.
I’ve seen mention somewhere that Amana recommends two-flute straight mills for routing slots because they have balanced load.
Here are some chip load recommendations:
I love my Speed and feed slide rule from Niagara Cutter!
I figure the 4mm mill is about 5/32" so I can go on the high end of 1/8" (4/32") recommendations. That chart suggests .003-.005" chip load for hardwood for a 1/8" mill.
- At 15K RPM, .005" chip load would be about 220 IPM or almost 5600 mm/min.
- At the 12K RPM you ran, .005" chip load would be 180 IPM or about 4500 mm/min
- 4000 mm/min would be about 160 IPM which would give 0.005" chip load at 10K RPM, and 3.6 cubic inches / minute if I step down 4mm / pass.
It’s a little scary to think of running 4000 mm/min but now I realize that is only 66mm/sec and I guess the numbers won’t lie. 5 thou isn’t really that much, and if I need to I can speed the spindle up to 24K RPM and get just over 2 thou chip load.
But I’m glad I bought 5 of those mills because I’m still worried I’ll make shards out of at least one before this is done.
I’d really love to hear what feed you used with that speed with the 4mm bit! Thanks!