I have Summon Monster IV on my spell list. Do I have to dismiss my eidolon before casting the spell, or can I cast it even with my eidolon out? I remember you couldn't have the eidolon and summon monster conjured by the SLA out at the same time, thus meaning that'd you'd have to dismiss the eidolon before using the SLA. I was wondering if this extends to the ordinary spell as well.
You do not have to dismiss your Eidolon to cast Summon Monster.
I can see your confusion, however. Summoners have a class feature called Summon Monster, which lets them use certain specific summon monster spells as spell-like abilities. When you use those spell-like abilities, you can't also have your Eidolon out.
The Summon Monster class ability says:
Drawing upon this ability uses up the same power as the summoner uses to call his eidolon. As a result, he can only use this ability when his eidolon is not summoned.
A summoner cannot have more than one summon monster or gate spell active in this way at one time.
Emphasis mine. This restriction only applies to the Summon Monster class feature, not the summon monster spell. There is no restriction against casting summon monster while you have your Eidolon out, or even when you have used your Summon Monster spell-like ability.