Before diving into the different resources to help figure out your 4th year, let's start with a quick inspection... of the various terms, acronyms, and shorthand you'll encounter:
Sub-internships, AKA Sub-I's, AKA audition rotations, AKA acting internship, AKA away rotation - Why are there so many names for basically the same thing? This is a special type of rotation, usually 2 to 4 weeks in length, where you will take on a role similar to an intern, literally a sub-intern. This is for residency programs to assess your abilities as well as your fit with the program's values and culture. You will essentially be "auditioning" for a spot in that program. However, that isn't to say you aren't entitled to judging how that program fits YOUR own values and priorities.
Visiting Student Learning Opportunities (VSLO), FKA Visiting Student Application Service (VSAS) - Again, why the different names for the same thing?? VSLO is a program developed from the AAMC where students can find and apply for Sub-I's. You will see during your search that the offerings will include different renditions of "Sub-I" (see above) after the program name, sometimes including just "elective." It is safe to assume that most the rotations you find there will be a sub-internship/audition, but if you are unsure, reach out to the program directly. Some programs will not have elective offerings through VSLO, you will have to apply to them directly.
Electronic Residency Application Service (ERAS), AKA your worst nightmare - ERAS is comparable to the AMCAS/AACOMAS application you completed to get to where you are now, except it's for applying for residency! (Have any regrets so far? I'm semi-joking.) Here is a sample worksheet that you can peruse briefly and subsequently panic over. (It'll be okay, I promise!)
National Resident Matching Program (NRMP) - The NRMP is an organization that facilitates the matching process for students and is what a majority of programs participate in. It uses an algorithm to match a program with a prospective student. There are of course exceptions, such as the military match and the San Francisco Matching Program. Their website holds a plethora of match data info, however, I especially like the "Charting Outcomes in the Match" Reports.
Medical Student Performance Evaluation (MSPE), FKA Dean's Letter - The MSPE is a standardized letter provided by a medical school to summarize and evaluate the academic performance, clinical experiences, and personal qualities of a medical student. Included in the letter are a student's Noteworthy Characteristics, more details found below.