What resistor will take you from 4.9V to 3.3V?

The nature of your question suggests that you are trying to convert 18VDC down to 3.3VDC Perhaps to power a microprocessor running at that voltage. If you just used a simple divider circuit then there would be two resistors involved, not just one. You cannot "take" 18VDC down to 3.3VDC with a single resistor. You could limit current but not "divide" the voltage down. Really a divider circuit is not the best way to do this either because you probably want to draw at least some current at the 3.3V level. A simple divider circuit voltage would be dependent on the load.. So... I would recommend either a switching or a linear regulator (non switching, and not very efficient). There are a bunch of DC-DC switching regulators that would do the job nicely... I like the ST Micro L5981. It will output up to about an amp at 3.3V and work from 2.9V to 18VDC input. You can find the datasheet at

http://www.st.com/stonline/products/literature/ds/13004.pdf

Oh, and I'm not pushing the STMicro solution, there are many other good regulators available from Analog Devices, Linear Tech, as well as TI, and National... So google a bit and you will discover many things for yourself...