Serious question:yes, the tank will fit, but you will need more Daytona parts than you would think. You will need the following: tank, subframe, undertray, tail section, tail light and seats. You will also need a Daytona 600 battery which is different than the S4 and you may need to modify the wiring harness to fit (ask Chrisjames - he would know). it is a good amount of work, but worth it if you like the look and if you can find the parts at a good price.
If that's it, then I bet the S4/TT600 battery could be coerced into the undertray. The only electrical component that would be changed in the swap would be the tail light and the turn signals. I have an S4 and just took the plastics off the tail when they got battered. I bought a D600 tail light which is just screwed into the back. I don't know if changing the gas tank would change anything electrically, but just the tail and its plastics shouldn't require a change of wiring harness.the battery on the S4/TT600 is a YTX12-BS while the Daytona 600/650 battery is a YTX9-BS. S4/TT600 sits upright while the Daytona 600/650 lays sideways.
from the Yuasa batteries site (http://www.yuasabatteries.com/vehicle_search.asp):
YTX12-BS Dimensions: 6" x 3.4375" x 5.125"
YTX9-BS Dimensions: 6" x 3.4375" x 4.1875"
i was never trying to imply that the wiring harness needed to be changed. what i was attempting to say is that the leads to the battery may need to be lengthened, depending on how the battery is situated. i tell you this because i know that the TT600/S4 and the D600 wiring harnesses are completely different. it was challenging to say the least to fit the D650 harness onto my S4. i almost gave up and put the Daytona subframe and plastics on, but with some luck and help from a good friend/extraordinary mechanic the D650 harness was made to work.but just the tail and its plastics shouldn't require a change of wiring harness.