As "Five-E", or "fifth edition"
As far as I can tell, abbreviations like "4e" and "5e" are unofficial community abbreviations for use in text, and there is no official canonical pronunciation defined in the books. These abbreviations are almost never used in D&D sourcebooks. However, we can use online sources to get a good sense of what pronunciations are most common in the D&D community.
YouTube videos are a good source of information on how words are pronounced. In my experience, among native English speakers talking about Dungeons & Dragons in particular, "3e", "4e" and "5e" are written abbreviations for what is pronounced "third edition", "fourth edition", and "fifth edition", respectively, although I have also heard these pronounced as written: "three-E," "four-E", and "five-E". I have sometimes seen e.g. "third edition" shortened to "third ed".
I've never heard "3.5" pronounced in any other manner than "three point five" in English.
"3.5e" I have heard pronounced less often, but "three point five E" is probably the most common, or occasionally "three point five edition"; however, these are longer and more cumbersome, so it's most commonly just called "three point five" in speech.
The examples given in Trish's answer, in regard to how Germans pronounce D&D edition names, I've never heard in English. I can't recall ever having heard "D&D three" (in English that would make it sound like a movie sequel) or "three-five"; in English, 3.5 is always said "three point five", read as it would be a number with a decimal point.
Examples from official sources
With these examples, I'll write the pronunciations of names as they are spoken.
WotC's Chris Perkins, interviewed in Between the Sheets: Christopher Perkins (1h40m):
I think by the end of fourth edition, there were 15,000 feats in the game.¹
Five minutes into RollPlay Presents: a 5E Roundtable Discussion (EP1), Adam Koebel uses both short and long form, showing that both terms are used when spoken:
When we think about fifth edition particularly, fifth edition Dungeons and Dragons what is five-E about?
At twelve minutes, Matt Mercer says:
I grew up for many years playing A-D-and-D second edition, I played three point oh, three point five, Pathfinder, I've gone back since, I've played first edition .. I ran a fourth edition campaign for a couple of years.
Fourteen minutes, Adam Koebel:
I'm actually a big fan of four-E ... When you were developing five-E, were there any of those mechanics that felt less like points of inspiration and more like shackles?
Koebel particularly uses "five-E", but Matt Mercer uses it too around 1hr14m:
Speaking honestly, the ranger in the initial five-E release ...
Puffin Forest's D&D 4e was a game:
The fact that I started with fourth edition is very unusual. Most people either started with three-point-five, or they were later and began with fifth edition.
I played a few sessions of three-point-five at the same time when I was playing fourth edition.
Matt Colville's Using 4E to make 5E Combat more fun! uses the long form in speech:
If you are a fifth edition Dungeon Master—and you are ...
I would just steal a bard ability from fourth edition. I would steal this ability; I would steal something like Fast Friends, which is a cool fourth edition bard ability.
This is the text for how to grapple somebody in third edition. ... This is how grappling works in fourth edition. And, by the way, I think this is is basically how it works in fifth edition.
However, in a written comment, Matt uses the abbreviated form:
One common criticism of 4E is that it "discouraged roleplaying." Now, certainly we had LOTS of roleplaying in my 4E game.
You might also find a good cross-section of the D&D community by asking on an English-language D&D forum, where multiple people can tell you how they pronounce it.
¹ Out of interest, Perkins' figure is an exaggeration; there were only 3,271 feats in the D&D Compendium by the end of 4e.