Hi, sorry if this has been covered before but i just cant seem to get it straight. So, I am wanting to build a 4x12 for my 150w Randall amp. Its pre amp is tube, and then it is a solid state. I am wondering what are the specs of the speakers i need to not damage either pieces of equipment? thank you
It needs a minimum of 4 ohms, so that's what you want to use as it will give the most output into the speakers.

Check out these links. They explain speaker impedances and wiring. You may want to read the top link first. The bottom link is just a bare-bones explanation of wiring.

Thank you, very helpful!. One thing though. Does wattage really matter or should i buy 150w speakers?
Or say if i wired 4, 50w speakers does it add up? (sorry for stupid questions)
I'll remind people about the columns where there is lots of info. Try this http://www.ultimate-guitar.com/columns/gear_maintenance/matching_speakers_to_amps.html

I don't know what you are using this for but I wouldn't worry too much about 4 ohms. your amp is solid state, the valve is just for a bit of tone shaping. Halving the ohms will give you extra power but not as much as you think and doubling the power only gives you an extra 3dB. in practice you will only get 2dB so only just noticeable. Most speakers for home builds are designed as 8ohm units and if they are offered in other impedances they are often not as good sounding as the original design. If it is for guitar then your Roland is going to be as loud as hell through a 4x12 so you aren't going to turn it up full at any time unless you want the rest of the band to hate you.

So long as all your speakers are the same impedance they will 'share ' the power evenly so you'll be fine with 50W speakers or above, you'll probably be fine with 35W speakers at 8ohms.