Personal experience - I just had our "meet the baby" party this past weekend (no gifts; just a party). DS was 7 wks old.
I could have had it sooner (we hosted it ourselves) but we delayed it due to the holidays but also due to DS's unpredictable fussiness. Having said that, 4 wks is a bit too soon IMO. OTOH, if MIL is hosting it for you, at least you don't have to prepare for it.
Couple things I would worry about:
1) How long will the event last? Mine was scheduled for 90 mins but DS was in such a good mood that I let people stay for 2 hrs. I think anything longer than that will be too taxing on a newborn, as they can get overstimulated and over-tired.
2) Do you plan to BF? If so, what are you going to do at the party? Go into a separate room to BF during the party? 4 wks can be a little too early to start pumping BM for a bottle.
3) How many people is your MIL planning to invite? We invited 15 ppl, and 8 ended up coming. It was a nice size group.
4) In terms of passing your baby around - since you are the mother, you have control of who can touch your LO! I asked everybody to wash their hands when they arrived. I didn't wear DS but either I or DH held him almost the entire time. We put DS in the swing for a bit but none of our friends touched him.
5) I know many people say you can "wear" your baby if you don't want others touching him. I have all the intention of wearing DS as well, but he absolutely HATES the K'Tan. Every time I put him in it, he screams and struggles until I give up. So, wearing your LO may not be an option!