my calculator says.... 300VDC (after being rectified from 240vac)
R1 = 13k
R2 = 680 ohms
Voltage out 4.5volts (from 300vdc) using a voltage divider, the resistive load is 280ohms, what ever that equates to in amps, 4.5/280 = 0.0160 amps
/ 0.0723 watts.
How's that sound? - but again, i still would not do it even if you told me it was safe :)
Firstly that 13k resistor will be dissipating 7W, so it better be rated for that power.
Secondly the load will not be a fixed 280 ohms if its a microprocessor, you will need regulate, so a 7805 or something would be needed. You need to check the highest current needed won't drop the input voltage to the regulator too low, and that the input voltage can't go too high either.
If you've got 240V ac anyway, a wall-wart is a simple safe reliable solution to the problem surely?