or can i run a 4 and an 8 together?
That's 2.66 ohms and is usually OK for most tube amps that can handle 4 ohms. The fact that the speakers are hooked up in series/parallel internally in the cabinets means that the actual voltage drop will occur at differing frequencies. Going lower than rated for a tube amp is usually fine. In fact most of them short their output to ground when no speaker is connected as a safety measure because... if you present too
high an output impedance to a tube amp (or output transformer coupled s/s amp) the transformer turns into a large feedback voltage generator and you get to kiss your output tubes, their sockets, and everything else in the immediate proximity goodbye.
An easy way to tell if your load is too low for a tube amp is if the amp starts distorting when you connect the lower than rated load, but sounds fine at a rated cabinet or the metal gray parts inside tubes start to change color and turn a shade of red or yellow. Going lower than rated does age the tubes much faster, but they live a hell of lot longer than most people give them credit for anyway. Usually if you're giving a tube amp too low a load and it's hurting it, the main fuse will pop before any major damage occurs.
