From 新完全マスター N1 文法:
〜といったところだ is a grammar point that has a meaning of 程度は最高でも〜で、あまり高くない. It's used to show that the quantity of something isn't very much.
From a dictionary of advanced Japanese grammar, p633:
といったところだ is a phrase that the speaker uses to explain something in a brief/rough/approximate manner. (Not necessarily any nuance about being not very much... Just estimated. "I'd say that it's about....")
(From a Mandarin textbook)
〜といったところだ can also be used to rephrase something using different/easier words to make it easier for someone to follow what you're talking about. Often used together with the ば form of verbs.
You can see that in both cases, the speaker is taking something that they think the listener might not know about/be able to grasp (woodblock prints of stars from the 17th-19th centuries, children's tastes) and then gives a more accessible example to draw a connection (photos of idols, curry and hamburgers).
That in mind, whether or not there is a meaning of "merely" or not depends a lot on context.
He could be saying "four or five of us will participate tomorrow, according to the schedule" without any nuance of a judgement about how many or few people that is.
If this is a monthly/weekly whatever meeting and there are normally ten people, then you can infer that there's more of a "merely" feeling involved.
So your first translation is the more accurate one - just know that, depending on the context, you might not necessarily have a "merely" nuance involved.
I'm not sure if they're interchangeable or not - ーといったところだvというところだ says they are, though.