When does Season 4 begin and What is to be expected?

Season 3 ends on October 31st, after that, there is likely to be a preseason before actually Season 4 starts. Pre-season is for all the massive changes they bring to the game and allow them to work out the kinks before they let competitive players run loose and exploit things they may have overlooked (AKA rebalance). However, ranked starts for Season 4 as soon as the MMR reset happens.

In season 3, the changes were quite extensive. There was a massive item overhaul, some jungle changes, some slight map changes, and some mastery changes. Here's what we know is coming in pre-season 4:

• A few new items. While nothing as extensive as season 3's item changes, where almost every item was touched in some way, these changes will introduce a few new items for specific niches, like a defensive crit item that builds out of hexdrinker and brawlers gloves for champs like tryndamere, called spellbreaker.
• Some map changes. This is mainly to combat the advantage blue side bot lane has in being able to take golems whenever they please. We don't know the scope of these changes. They may change the jungle extensively, or only slightly.
• Mastery changes. Feralpony was on the forums asking for feedback about whether masteries should be more/less effective than currently. IIRC, he wants there to be more differentiation between the three mastery setups, with scaling masteries that provide a bigger bonus lategame being put lower in the trees so that you have to invest a lot of points to get to them rather than just putting them a few points in at the top. He also wants to do something about the improved summoner spell masteries (either remove them and move their bonuses to the spells themselves or improve the masteries themselves, can't remember what).
• There will definitely be a mega patch that will probably hit at the start of preseason with a major lore update for the shurima region, much like how freljord had a major patch recently. This update will likely include: lore updates from the champs of that region, skins for champs of that region (scorched earth renekton has been on PBE for a while), ability rework for Xerath (a shurima champ), visual update for Sivir (shurima champ after lore rework), and magma chamber (the 1v1/2v2 map showcased at the all stars in shanghai).

Riot's soft reset is: (MMR(old)+1200)/2=MMR(new)
Example: (1600+1200)/2=1400
Example2: (1000+1200)/2=1100