The core rules of Warhammer Fantasy Roleplay 4e are very much the same as WFRP 2e. 4e keeps a few things from 3e like pools of points for fate, fortune, resilience and resistance. The cards, counters, tokens and trackers are gone but some of the iconography survives in the design of the core book.
Combat is more deadly with the option to roll critical hits not just on double 00 but on any double result that meets or exceeds the target number. A system of advantage is thrown in as well as levels of success that effect the outcome.
Skill test now offer opposing rolls and checks can be a simple or a dramatic test. Dramatic tests allow for different levels of success that increase the favorability of the outcome.
Many of the innovative effects and conditions from the 3e cards have been incorporated into the core rules. Many more rules are listed as optional.
The stats suffer from inflation across the board when compared to 2e. You’ll need the extra resilience to survive the eight pages devoted to critical hits, broken bones, amputations and the other grizzly fates that await your PCs.
The career system gets an overhaul. Many of the career paths in 1e and 2e could top out early leaving characters with no real choices for advancement other than starting a new basic career. The career paths are now a better fit for campaign play.
If you have the bulk of the 2e material the 4e core book is still worth having if only for the many optional rules that could be incorporated into an existing 2e campaign.
As far as adapting an existing campaign you would have to recalculate wound totals. The spread of the stat scores is wider than the norm under 2e. There is not a complete overlap of all careers and skills but the flavor of the game is very much the same.