OP, from the perspective of the adcoms, there is really no difference between being listed as a fourth author as opposed to the third. And contrary to popular opinion, the last author is usually the most senior person (the PI) who secured the grant/funding to conduct the study and who heads the lab in which the research was conducted. So, the first author and the last author are generally viewed as the most significant among the scientific community. Anyway, congrats on the publication!
Alright, so based on what I know, I'd slightly modify this. The issue is where you are in your career. Anything prior to establishing your own research "lab" (which can include a basic/translational research lab, or you taking your own responsibility doing clinical research and being the primary mover on it), you're basically going to be anything from 1st to penultimate author. Once you have your own lab, you'll be sponsoring/mentoring/directing the research with someone below you taking on the primary work. In this case, you'll be last author or senior author. For clinical research, this often is either the clinical research director, the director of the division or the chair of the department, depending on local politics.
For college students, med students, residents, and fellows, you're looking at 1st author being the best, through to second-to-last author. Having any publication by the time you apply to residency puts you in a minority category, so author # means little in the grand scheme of our careers.
Now, what is always lost in these threads is how much work you have to put in to get an authorship. In my view, you have to be the original creator of the research idea to be first author, and do some meaningful portion of the rest of the work for the others. Sometimes, the chair or head of the lab will come up with the idea but really let you independently figure out how to answer it - what assays to run, what antibodies to choose, or, how to design your clinical study, what patients to include/exclude, etc. This likely is worth 1st authorship as well. In these instances, who gets authorship is relatively unambiguous - a good way to go about it as a trainee.
Middle authors should technically be assigned by how much work was done, but this isn't always the case. There's more politics involved here. Also, it's difficult to order a med student tediously reviewing 100 clinical charts and a resident who helped organize the data, run statistics, and write key parts of the paper - who deserves 2nd author in this case? Really, the senior author should be managing this in an honest manner.
Keep in mind, research is inherently tedious and there are a lot of parts of it that we as trainees are privy to and a lot that we're not. So try to contribute to a project, know the project - both the parts you did and the parts other people did, hopefully engage in an interesting discussion about it during an interview, and continue on your way. Learn about how authorship works so that you can put yourself in an opportunity to become 1st author (and do all the work associated with that opportunity) but don't worry about it when it comes to your CV. You're just going to do more harm than good.
