Four great options.
Do you want to study business, economics, math, physics, other humanities, or ???
Olin has a high rate of job placement overall among the top 100 or so undergraduate business schools (currently ranked at #6); however, I am not referring just to IB placement. Great business school with lots of major concentrations as well as the MBA program on the same campus.
As an elite National University located in the Midwest, the campus culture is likely to be noticeably different than the campus culture at your other options.
Bowdoin College will be the smallest school so you need to consider whether that will be a plus or a minus among your preferences.
Middlebury College places the most students per capita in your desired industry of IB.
Wesleyan University is a great school if the fit is right–just like your other LAC options.
Was NYU-Stern an option for you ?
If you want to study business, then the choice is clear.
Cannot recommend one LAC over the other because personal fit is very important at a small school in a fairly rural area (Middlebury & Bowdoin).
WashUStL Olin offers meaningful internship/externship team based experiences for credit.
Without knowing whether or not you have a strong preference regarding major area of study, it is tough to recommend one school and always difficult to direct one to a small LAC if that individual has not visited the campus while school is in session when other larger options are equally as attractive.
In short, need more info. regarding your preferences as all 4 can result in getting an IB internship.
P.S. Not sure if things have changed within the last two years, but I know of several individuals who did not get IB directly after finishing college, went to work in the consulting industry (mostly Big 4 specialty divisions) for a couple of years, then moved on to IB. Please keep this in mind as there is more than one path if IB is your absolute preferred career goal.