Please clarify what you mean by "unstable." The output impedance of a tube amp is generally higher than a solid state amp but I don't think it can accurately described as being unstable.
The 4 ohm tap will provide less power than the 8 ohm tap so if you listen at loud volumes that may be a factor. There is no danger to the amp or speakers.
The 4 ohm tap will provide less power than the 8 ohm tap so if you listen at loud volumes that may be a factor. There is no danger to the amp or speakers.