Those type of amps will inherently sound different.
The 4 is a single-ended Class A amplifier, and the 10 is a push-pull amp. I don't want to (as others have done) say that the 10 is a 'better' amp it's an apples to oranges comparison. That's true on the 4 also, but at least with that amp you can drill two new holes and just move the transformer out of the way. Also, the internal layout of the 10 is a bit stupid the power tubes are almost right up against the speaker, so your choice of replacements speakers will be limited. Looking at the PCB, the circuit of the 10 looks more complex than the 4, and even has a piggyback PCB with ICs on it - I have no clue what the ICs are for.
That said, the 4 still sounds good, but might not be the 'classic Vox' tone people are looking for. The AC4C1-12 sounds smaller, even with the larger speaker. I used to own an AC15C1 and the 10 sounds more similar to it than the 4 does. There also seems to be a little more preamp gain available on the 10. Not just volume-wise (the AC4C1-12 is actually quite loud), but in tone - there is much more bass with less emphasis on the mids on the 10.
The AC4C1-12 has a 12' speaker, and the AC10C1 has a 10' speaker however, the AC10 for whatever reason sounds much larger than the AC4C1-12. This is an old thread, but I currently have both amplifiers and I wanted to add my two cents for others to reference.
