Resistor values for a 50 ohm pi attenuator network - equal source and load
Perhaps the most common use of an attenuator is in 50 ohm radio circuits. To this end I have included a small table in figure 2 below which depicts common power reduction values of 3 db, 6 dB, 10 dB and 20 dB.
What you want here is 6dB (reduction by 4). Reduction by 2 would be
3dB. (If you want to increase your range by 100%, you usually need a boost
of signal of about 6dB, which is 4x more power).

Resistor values for a 50 ohm pi attenuator network - equal source and load
For 4W to 1W, R1=150 and R2= 39. 2W resistors should handle it.
To calculate other attenuation values:
What is this "K" factor in the formula for a pi attenuator network
The factor K is called the ratio of current, voltage, or power corresponding to a given value of attenuation "A" expressed in decibels. It is the more difficult calculation which proceeds as follows:
"K" is the number 10 raised to the power of the value of attenuation "A" in dB, divided by 20.
K = 10 ("A" / 20)
As just one practical example, let's look at 3 dB attenuation and calculate our "K" factor.
K = 10 (3 dB / 20) = 10 (0.15) = 1.4125
On my calculator I entered 10 and pressed YX then entered .15
Entire explanation is here:
http://www.electronics-tutorials.com/ba ... uators.htm