To say a synth is out of date based on the number of voices is a complete misnomer in my view.
Look at the amount of monosynths out there - I don't think you can say they are all out of date because they only have one voice! Further, look at the huge resurgence of modular synthesis over recent times. Most setups tend to have one or two voices, I don't think anyone would say these were out of date.
Ultimately a synth is a musical instrument, or at least a way of creating sound. You need to simply decide what you want out of it.
I own 2 synths - an Analog Keys and a Prophet 08. The Prophet is an 8 voice polysynth with quite 'traditional' architecture. The AK is 4 voice of course, but has incredible sequencing and performance potential. I use these for very different things.
If I could own 2 other synths I would get a Moog Voyager (1 voice) and a Vermona Perfourmer (4 voice) - because the sounds and capabilities of those are different to what I have. It's an expensive business and thus a bit of a pipe dream, but I have played them both and find them very inspiring for very different reasons.
So what you must decide is what is important to you, what do YOU want from your synth. There is no 'best' synth, just as there is no 'best' guitar or piano for example.
If more voices is what you need for the music you want to create then sell up and get an 8 or 12 voice synth. I totally understand (and I think pretty much any musician has this when starting out) that it is initially difficult to know what to get, and it is tempting to look for the 'silver bullet' instrument which is the 'best' and 'perfect' one - it simply does not exist.
My advice would be to explore what you DO have. Learn it, get creative - you'll find there are magical ways to get stuff done. Then once you have mastered it you'll know much better whether you want to get something else or not.
Just my 2 pence worth!