Riot looks to fix those advancement problems by amending the safety net at the bottom of every league.

The problem of wildly varying skill levels across individual leagues and divisions made advancement uneven and sporadic, and the problem only got worse the higher one climbed as more and more people who were honestly in leagues above their actual skill were being squared off against players struggling their way to the very top.

The first thing Riot will be doing in Season 4 to fix these issues is to remove league point clamping.  Clamping is where the game grants increasingly small LP rewards to players as they approach the promotion series to advance into a new league, with the intention of preventing players from getting a string of several lucky games and suddenly being raised to a league they should not be in and cannot be removed from.  In practice, it has led to a great many players feeling extremely frustrated, having 90+ LP and still only gaining one or two per win while losing 15 or more on a loss.

Buyer beware, you might have to pay more than once in Season 4.  Or banned.

The second thing Riot is doing is removing immunity from dropping a full league if a player loses enough games.  They found a noteworthy number of players were advancing to division V in a league and then losing consistently, growing steadily more toxic as they lost more and more frequently.  

Players will have a solid 10 games of full immunity on reaching a new league, but if their matchmaking rating drops far enough, the example given of dropping a full league (a Gold V player playing mostly with Silver V players, for example) they will get a warning they are close to dropping to the next league down.

Use the rest of the season to practice and get in shape, everyone!  No free rides in Season 4 for ELO-boosters!