I would be careful in advising our friend not to get a 4x12. Why? Because it's all about the sound he wants. A 4x12 cabinet has a unique sound and certain thump to it compared to 2x12, open-back enclosures, etc. You can use that 4x12 with any amp just as long as you match impedance. I agree that portability is an issue
For instance, I'll use a 4x12 with a Fender Super Champ combo or my Traynor Bassmate combo. These are both tube amps that are < 20 watts and I am just using the amp portion of the combo. The sound is very full and a 2x12 doesn't really compare. It's important to note that the sound level is not ear-splitting either.
Klash, as for the explanation, here's my take. A 1/2 stack is a head and 4x12 cabinet. A (full) stack is a head and two 4x12 cabinets. I think you're looking for a 1/2 stack. It's important to make sure that the head and cabinet impedances are matched (IE: 8 ohm head to a 8 ohm cabinet). Most heads have a selectable impedance (4, 8 and 16 ohms). Cabinet wattage rating is how much power it can handle and is the product of # of speakers x lowest wattage rating of any individual speaker. (IE: 4 x 75 watt = 300 watt cabinet). Lower wattage equates to earlier breakup as mentioned in the previous post, but you have to be careful not to exceed the overall rating of the cabinet or you run the risk of blowing it. That's why cabinets have a higher power rating than heads.
Not to complicate things, but if you have mixed speakers in a 4x12, like 2 - 30 watt and 2 - 75 watt. The overall power handling is 4 x 30 (lowest wattage speaker) = 120 watts. Won't bother going into the theory, but that's what it is.
Make sure you know what sound you want and at what sound level you want to be playing it at. 50 tube watts through a 4x12 is seriously loud.