Being gummy copper tends to form a long chip that does not curl up into a tight coil. A two flute cutter has more space between the two cutting edges so the chip has a better chance to curl out of the flute. With a four flute cutter the chip can hit the back side of the preceding tooth because the flute is smaller, and then the chip just wedges into the flute.
If possible use work hardened copper for machining; it will give a better chip than annealed copper. Also use a lubricant, tapping fluid brushed along the line of the cut can help considerably. Keep the rpm up, probably the maximum your machine can run, and experiment with the feed to try and obtain a chip that comes off in a thin wafer.
An open mind is a virtue...so long as all the common sense has not leaked out.