In my opinion, yes...and an emphatic YES at that!
I had hoped that I'd be buying the S910 and saving some substantial money. I had the Tyros3 here for over a year and although it is a magnificent instrument, and a healthy upgrade from the old Tyros2 (more features, beefier sound to name two) it lacked the "sweetness" I liked that was present in the latter. No amount of tweaking the Tyros3 could give me that sweetness, and nothing I could do with the Tyros2 could give it the stronger sound of the Tyros3.
So, the S910, definitely had the "sweetness" of the T2 and, although it lacked the strength of the sound of the T3, it had several features of the latter that I really liked, and, it was considerably less expensive, so I figured I'd be pretty happy with the S910.
Then the Tyros4 arrived...and my plan slowly deteriorated with every minute I played this great new instrument.
Yamaha wisely saved a lot of money by not going with a new/different cabinet, and used it to do substantial upgrades to the sound engine, and, of course, to add those great new vocal patches, more SA2 and Mega voices, more new and re-worked styles, and other nice features like MP3 playback and a much larger 250G HD.
The Tyros4 has the sweetness of the Tyros2, and the strong beefy sound of the Tyros3, and comes out of the gate sounding considerably better than both of them, in my opinion.
Part of me wishes it didn't, because an S910 is a good saving, and perhaps in your case, if the money is big consideration, the S910 will continue to serve you very well, indeed...I still like playing the one here, but after playing the Tyros4, there is no contest...and no going back for me.
The people with Tyros4 that I have been in touch with, are every bit as impressed as I was...and, it sounded awesome right out of the box....I haven't tweaked the instrument one iota, and has a wonderfully strong and powerful sound, as well as a soft and delicate tone when needed.
As with any big purchase, I strongly recommend playing the instrument for a good amount of time, or if that's not possible, there should be an agreement to be able to return it if you don't find it exactly what you want, because spending this much money should give you total satisfaction.
I had other uses for the money, not all of them just for increasing the pleasure with my other hobbies, but, I believe the investment in the Tyros4 is worth it, in my case.
Good luck with your decision, Beach, and I hope you get maximum enjoyment out of whatever you decide to get.
Ian
PS...yes, I work for Yamaha, but the reasons I do are because I love their sound, and I love how they do arrangers...it was my choice to work for them, and it was based on those grounds. And Beach, I'd be a total fanboy, even if I never had any other relationship with them other than just playing and enjoying the instruments.
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.