Siblings in Audioengine's HD series, so they share a family sound. It comes down to size, output and price.
| Area | Better | Why |
|---|
| Bass | A | Audioengine HD6 digs deeper - 50Hz against 60Hz. |
| Built-in power | A | Audioengine HD6 has the bigger amplifier (100W vs 60W RMS). |
| Connections | A | Audioengine HD6 adds digital inputs. |
| Size | depends | Audioengine HD4 is easier to place; Audioengine HD6 is bigger, which usually goes with fuller bass. Your room decides. |
| Expandability | depends | Audioengine HD6: a sub output; Audioengine HD4: a sub output. |
| Price | B | Audioengine HD4 is cheaper - $449 against $699, about 36% less. |
A Audioengine HD6 B Audioengine HD4
depends neither is better in the abstract - it turns on your room, your system or how you will use them. Only specs published for both speakers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
The usual trade-off: Audioengine HD6 measures better, Audioengine HD4 costs about about 56% more% less. Whether that gap is worth the money is the call only you can make - and in a normal room it is often smaller than the numbers suggest. If a subwoofer is in your plans, the bass gap largely stops mattering - you would be crossing over above 60Hz anyway, which makes the cheaper Audioengine HD4 much easier to justify. In a large or open-plan room, Audioengine HD6 has the cabinet volume to fill it more easily; in a small room, near a wall or at desk distance, Audioengine HD4 is the easier speaker to get right.
And remember what a spec sheet cannot show: how each looks in your room and whether the finish suits it, what dealer support and warranty are like where you live, and how they actually sound to you. If you can hear them first - ideally with your own music - that will tell you more than any number above.