| Just to expand a bit on what others have said...
You probably mean that the head is stable under a 2 ohm load. A nominal 2-ohm speaker load is very low-resistance (e.g., two 4-ohm speakers in parallel). The lower the current resistance, the greater the current the amplifier must supply in order to maintain a particular voltage gain. Over-drawing current from an amplifier by reducing load impedance can cause catastrophic thermal failure. So if a solid-state amplifier is rated at a 2-ohm nominal load, then it is safe to operate at a nominal impedance of 2 ohms or higher. (So the 4-ohm load would be fine.)
If you're talking about true tube amp head, then the situation is a bit more complicated because they use output transformers, which typically need to be impedance matched to the speaker load to avoid reflected power (which can be destructive). Most tube heads have an output transformer with multiple taps at different winding lengths, and a corresponding selector switch to toggle between taps depending on the cabinet configuration in use. I've never heard of a tube head with an output transformer fixed at a 2-ohm impedance, so my guess is that you're talking about a solid state amplifier. | 
