I think it will be very capable of cutting that deep with no problem just watch the speed and feed
I accidentally ordered a long 1/4 in mill and it is 4 inches and I only have 4 inches under the 660 so I can cut paper or real thin foil at some point it will get used but I was bummed when it showed u p and it was so long but only 18$ I think, amazon delivered in1 day but 
since I started using speed control 1/4 inch is no problem can’t wait for the PID for 660 Peel pocketing works great 4-8mm dc and 800mm/m (probably would work even deeper I just have not had a opportunity to try it) slotting not so deep 4mm good 8 bogs down PID may help that this MACHINE is incredible find your sweet spots with the material and it is a dream come true I love my MPCNC The Low Rider 2 may be in my future also ;-0