According to the RAW, no.
If you run into an evil cleric, who summons a fiendish girallon, you don't get extra XP. If the cleric had cast a different 4th-level spell, you don't get extra XP for that.
If you run into a demon, no - the extra summon is included in the monster's CR.
Having said that, the RAW are sometimes broken. If you face a succubus, she has a ~35% of summoning a balor, which will kill opponents of her CR level. For extreme circumstances like that, yes, the DM should give a hefty XP reward. In fact, WotC should not have put the succubus into the MM like that. I hope the succubus gets fixed in the MM 3.5.