I've been asked several times how V5 will differ to V4, and I'm sure I will be asked again. The short answer is: there's no short answer!
V4 was a product of its time and is inevitably showing its age as additional sources have since surfaced, been analysed, debated, and subsumed into the author's best understanding of the original game. While V5 encompasses much of that nuance, it remains broadly the same game, if somewhat more faithful to the source materials.
The most obvious additions to V5 will be:
The illustrated version,
The annotated version,
The addition of a book 4 (of appendices).
Book by book, the major changes/improvements I can think of are:
Book 1 (Heroes & Magic)
Remains largely similar to V4. Some minor nuances include:
V5 brings back player level titles,
Halflings have become Hobbets (with an e), and have the (more authentic) advantage of two shots counting as three.
Elves with magic weapons are more awesome, as they were meant to be!
Spell lists revert to the shorter M&M set (with V4 additions moved to the appendices).
Book 2 (Campaigning & Adventures)
Will be way better organised around the referee's actual workflow (preparing for campaign, running the campaign, running adventures, running combat). Content changes include:
Additional content from Dave Arneson's First Fantasy Campaign (re: preparing and running the campaign) that is sorely missing from UWA.
V5 combat is expanded, but is not so much "changed" as it is spelled out more clearly. This is (arguably) a major part of the V5 value proposition. Specific expansions include:
-- V5 leans more into Fighting Capability (Man, Hero, Superhero) as a combat stat, which was only hinted at (by normal, heroic, superheroic language) in V4.
-- V5 is explicit about melee rounds (which V4 glossed over),
-- V5 includes guidance on per-melee round movement (which V4 omitted),
-- V5 is explicit that declarations are only required where a spell caster could be interrupted, whereas V4 has declarations generally.
-- V5 describes deterministic initiative with a d6 fallback, whereas V4 relied on referee fiat with a d6 fallback. (I'm still on the fence whether to revert to V4 and push deterministic initiative to the appendices).
-- V5 has two d20 attack matrices (men attacking, monsters attacking) per the original, rather than V4's single combined table. They achieve much the same.
-- V5's parry is closer to the original with a −2 penalty and weapon-breaking, rather than V4's −4 penalty and disarming.
-- V5's jousting is nearer to the original rock-paper-scissors mechanism.
Other book 2 changes are mostly minor refinements that pull back some of V4's (intentional at the time) flourishes. Anything substantial will be covered in the V4 compatibility appendix.
Book 3 (Monsters & Treasures)
Book 3 will be trimmer than V4, with a bunch of V4's expanded monster descriptions pushed to the appendices. Otherwise, it's a very similar collection of reference material.
One notable change is that V5's numbers of monsters appearing are calculated per FFC and UWA, rather than the fixed number ranges appearing in V4 (and M&T).
Book 4 (Appendices)
Overall, V5 pushes a bunch of "extras" (over and above the original sources) into a book 4. These include:
the thief, V4's extra spells and monsters, an expanded equipment list, additional nuances of the normal/fantastic combat system (including the 2d6-based matrices),
A list of "house rules" to maintain backward compatibility with V4.
That is probably not exhaustive, but should give a feel for it.
Hope that helps :)