Elo reset?

Nobody knows ;)

soon.

When 4th Season started.

I would guess it will happen very soon, mid january-early february. The reset will only be soft, which means that your elo will be reset towards 1200, which is the "start elo. So if your MMR is 1800 for example you will be set to an MMR of 1500. Now you only have to calculate that into leagues and you know exactly what will happen :P

Within 2 weeks.

No one knows, but pretty soon i would say.

Not too unlikely that it would start alongside the start of the 4th season of LCS, which is the 14th of January I believe.