Could someone tell me how compatible the L5R 4th Edition is with the 1st Edition source books? I have the entire 1st edition (every book they released) and am planning on dusting them off to run a campaign, but would like to use the 4th edition rules. Will I find the 4th edition changes all my NPC stats in the 1st edition books I have? If it is going to take a massive effort I will just stick with my 1st edition material.
On their own? Not very. The same concepts exist, albeit at different points in the timeline. The rings are ostensibly the same, and for the most part the same skills exist with some minor differences. However, the rolling mechanic is different (despite still using a variant of Roll & Keep), and pretty much all abilities (ads/disads/schools/spells/kiho/kata) are quite different. Some of your favorites still exist, though so you just need to read thoroughly, especially since some of them are included in other sourcebooks than the core.
There are guidelines on how to update your old characters, but usually it's just easier to start anew and use the old sheet as a reference. After all, the GM can award extra points for character creation as they see fit to keep the characters at the level they deserve to be.
As far as comparing any edition to 5e, the only thing that really matches is the setting. The game has aspects that are quite similar to Star Wars: Edge of the Empire. There are d12 and d6 to roll, and depending on the symbols that turn up, you determine the results. The rings still exist, but how they are rolled is more abstract. You use the ring that the GM considers to be the closest theme to your intent, and if the dice turn up the right symbols, you add "raises" after the fact. Additionally, progression is no longer done by numerical insight. Depending on your school, you must have a certain collection of rings, skills, and abilities to achieve the next technique level. The new rolling mechanic means a drastic change in every ability. You definitely need to start from scratch on any of your old characters to make them fit the new system.