Creating a 4th order Butterworth filter R
do I need to change the number to “2” so that it is a second-order

Yes

and should I also double the cut-off frequency i.e change “W” to 0.3 from 0.15.

No
Strictly speaking, doubling up the filter will make the gain at the cutoff frequency -6dB instead of -3dB. So you have created a Linkwitz-Riley and not a Butterworth filter.
If you want a gain of exactly -3dB at 150 Hz you would need to increase the frequency slightly. You can work out the math by calculating the -1.5dB point of a Butterworth filter, but trial and error is probably quicker. 185Hz should get you fairly close.
In any case, the shape will still NOT be that of a Butterworth. Cascading two Butterworth filters does not result in another Butterworth filter. The zeros are in the right location but the poles are simply duplicated and are not Butterworth-distributed any more.