cporres,

I also live in a city at 4,000ft asl and I am using a Saito 100 in my Excelleron.
The saito 100 is marginal for an Intermediate pattern at this altitude, I've tried it at a contest at 4,500ft and it definitely lacked power, but then the Saito is a lot less powerful than a YS110, so I would think that a YS110 would be perfect at 4,000ft, unless you intend to do 3D.
I've also heard a YS110 is more powerful than a YS120, although I haven't tested this myself.

Gabriel