yes, however you mentioned a 4S lipo which is 14.8V . I haven't seen or heard of a servo that can handle that, only 7.4V servos. Now, that DOESN'T mean that no servos that can take that exist, but I just hadn't heard of any so I assumed you mean 7.4V .
If you are running 2 7.4V lipos, yes technically you can, however, personally I wouldn't as now you have one lipo getting drawn harder than the other and can get some weird balancing issues etc.
Best bet would be to use an external BEC in my opinion that you can dial in the voltage you want.
We used to do it all the time back when 1/8 electric got started to have a cheap ESC (the Mamba Max) that could handle 4S usage.
I would contact Tekin and see what they recommend on using a BEC with there RX8.
Personally though, I haven't see the need for more than 6V using a servo that has the specs that I mentioned at a minimum.
A nice very high torque servo with decent speed on 6V is this one:
http://www.servodatabase.com/servo/hitec/hs-5955tg
I just sold one as I got a faster servo, but the guy who got it is extremely happy with it.