The number one thing you want to do in any short course is get students interested and motivated - you can convey very little information in 3-4 hours, but you can motivate your students to learn more. I'd recommend picking one topic of interest to your community and showing them how R can help them kick butt in that area. Cut ruthlessly - you want to figure out the absolute minimum path from knowing nothing about R to being able to do something useful, something that makes your students say "wow, that's cool". For me, I use graphics - in 3 hours you can teach the basics of ggplot2 (scatterplots, histograms, aesthetics and facetting) giving students a powerful toolkit for data exploration.
I would recommend using RStudio. I wouldn't recommend talking about code style, vectorisation, or probably even for loops.
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