My current 4th edition Dungeons & Dragons campaign is scheduled to end soon, after running for 3 years. I am planning on a big, new campaign which will also be basically 4th edition Dungeons & Dragons, with some modifications from 5th edition, which is why I call it my 4.5E campaign. But maybe we might first run a few session of the 5E starter box, just to give everybody the chance of an informed view of which edition is most suitable for our group.
A real D&D 4.5 taking a 50-50 mix of the best of the two editions in my opinion isn't possible. 4th edition Dungeons & Dragons and 5th edition Dungeons & Dragons are largely incompatible where combat and stats are concerned. 4th edition has more levels, and your various numbers (e.g. attack and defense stats) go up by a lot. From level 1 to max level in 4E your attack goes up by AT LEAST +15, which is the "half your level" bonus, and not counting stat increases and the assumption that you'll wield magic weapons. The equivalent increase in attack for 5E is only +6; That is deliberate, a concept called "bounded accuracy", and makes it easier to use for example monsters over a wider range of levels. An orc in 5E doesn't become trivial just because you gained 3 levels. So 4E and 5E have an incompatibility based on very simple math, and you can't easily use combat rules or monsters or magic items from one system in the other. There is no such thing as a compromise, a real 4.5 edition which would be somewhere half way between the two. The math just doesn't allow it. Given this incompatibility, I believe that my group much prefers the more tactical combat of 4th edition over the somewhat more random combat of 5E, and so if I have to choose one combat system of the two, I believe that 4th edition for this specific group is the better choice.
That doesn't make 5E a bad system, and in fact it probably is a better system for new players. But one of the disadvantages of 4E is that it is hard to learn (and consequently slow if you haven't learned it well), and my group is already past that stage, so this isn't much of a problem for us any more. That brings me to another disadvantage of 4th edition, which is on the role-playing side. 4E rules are very focused on what your character does in tactical combat, and aren' all that explicit on the role-playing aspects between encounters. That is most visible in character creation, where a 4E character is a basically just a bunch of stats and powers, while a 5E character is far more fleshed out from the personality side.
Fortunately role-playing isn't based on math. Which makes the 5E personality creation system compatible with 4th edition. What I jokingly call D&D 4.5 thus is 4E with characters created using a modified 5E personality traits system. And I'm throwing in a bit of 13th Age in for good measure.
In 5th edition a character has personality traits, bonds, ideals, and flaws. For my system I'm taking the bonds, ideals, and flaws, and replace the generic 5E personality traits by 13th Age's "one unique thing". I am also using a system of backgrounds which will be a mix of 4E "themes" and 5E "backgrounds". And finally I am going to use the inspiration rules from 5E, giving players a bonus roll reward for good role-playing of their personality.
The "one unique thing" is a concept where you ask each player to come up with something which is more or less unique to them. That can range from classical fantasy "I'm the lost heir to the throne of XXX" to weird stuff like "I have a clockwork heart". The player can propose pretty much anything, but it is up to the DM to translate that into rules. For example if a player tried "I am invincible" as his unique thing, I would rule that he *believes* himself to be invincible, without actually having any immunity to damage. The one unique thing should add a lot of flavor to a character, but not really change his power level. I like this one unique thing concept more than I like the more generic personality traits of 5E (example from the starter set: "My flattery makes those I talk to feel wonderful and important. Also, I don't like to get dirty, and I won't be caught dead in unsuitable accommodations.").
For bonds I am slightly expanding the concept from what is described in the 5E rulebooks. It remains a description of what organization the character has a bond to, for example like the "houses" in Game of Thrones, or a location like a home town. But in the campaign I want to play there will also be moments where the players have to talk to connections, people they know, people who owe them a favor or whom their owe a favor. So I'm adding those connections to the bonds category, and that is something that can grow over the campaign.
The ideals and flaws concept I'm taking straight from the 5E rules. What does the character believe in? Where is he vulnerable or at least not perfect. The flaws are the one personality aspect which is the most likely to be rewarded with an inspiration bonus if role-played, because it doesn't come easy to everybody to play a flawed character. The inspiration rule is also straight from 5E: You can only have one inspiration bonus, and can't gain another until you used the one you have. Using the bonus means *in advance* of an important dice roll saying that you want to use your inspiration, and then roll two dice instead of one and take the better roll.
My planned "4.5E" campaign will be one with a big campaign story. That necessitates a certain amount of willingness by the players to follow the given story. But obviously I don't want to turn that into some sort of cinema in which the players are spectators. By encouraging them to freely choose different aspects of the personality of their characters I hope to get lots of interesting personal side stories, as well as adventure hooks. At least that's the theory, I'll have to see how that eventually works out.