I have to agree with Nishedha 4mm is large, I posted the image below a few months back, below is the description.

I also knew an American bead maker John Christensen Indian name "Keokuk" who prided himself in the fact that he made HISHI(flat faced beads), in very small diameters, if I remember correctly his hole size was less than .5mm, he would string the beads in ten to twenty strands for his necklaces ........ Truly amazing ...... Truly beautiful. I attached an image of ear rings by John I just found on Ebay notice there are five strands on each ear ring and I would guess the ear wire is 20 guage .032 inch so the bead size looks like it would be about 1.5mm

I think there is no size limit (small) for beads to be called beads, if you can put a hole in it and then a thread through the hole I'd call it a bead.

FROM EARLIER POSTING
The image below is a choker I used to wear, all beads on this choker are beads found on ant hills, the smallest forty one came from the ant hill mentioned above. They measure from 1.72mm to the largest at 2.96mm.I personally have made beads of this size and smaller, I have modern tooling, they did not.

All my best ... Danny