I have a variety of collets including R8, 5C, 2S, and a number of specialty collets for drill grinders ,and tapping heads. They all have 3 slots 120* apart except for the drill grinder collets I made in the shop. In my opinion the 3 slot collets have better gripping force due to the larger face on each section, and less propensity to collect chips since there are fewer slots and they are wider.
The 4 slot collets I made work fine, but there isn't much force applied when grinding drills as opposed to actually drilling or milling a hole or slot. When purchasing collets be sure to look at the specifications. My 5C Hardinge collets have a clamping range of +.007" to -.002". The cheaper Shars ones have a clamping range of +.000" to -.008"
As for the Hardinge clamping range you can clamp down on smaller diameter stock, but you run the risk permanently deforming the collet. As mentioned in another thread it isn't wise to try to respreads the fingers. In all likelihood you'll break the collet.