Assuming that your tube amp has output taps for 4 ohm speakers, it will deliver the same power(watts)into 4 ohms as in to 8 ohms. But that is not really good, because the typical ss amp would deliver twice as many watts into 4 ohms. Loudspeaker SPL ratings these days are usually given for a certain voltage applied to the speaker. The voltage used is 2.83 volts and this corresponds to 1 watt for a 8 ohm speaker and 2 watts for a 4 ohm speaker.
So, to answer your last question...4 and 8 ohm speakers with the same SPL spec will play equally as loudly if driven by a ss amp, because the ss amp will deliver the extra current required for the 4 ohm speaker. If a tube amp is used, the extra current must be provided by using a lower tap on the output transformer, which means that output voltage swing has been traded off for current. Therefore the output voltage capability, which defines how loud the speaker plays, is reduced. So, in your case, you will see lower SPL with a 4 ohm speaker. But it might be ok anyway.
So, to answer your last question...4 and 8 ohm speakers with the same SPL spec will play equally as loudly if driven by a ss amp, because the ss amp will deliver the extra current required for the 4 ohm speaker. If a tube amp is used, the extra current must be provided by using a lower tap on the output transformer, which means that output voltage swing has been traded off for current. Therefore the output voltage capability, which defines how loud the speaker plays, is reduced. So, in your case, you will see lower SPL with a 4 ohm speaker. But it might be ok anyway.