This is an excellent question, OP. While athletic scholarships are not absolutely "guaranteed" for four years at any university, at some schools a coach is not permitted to reduce or terminate a scholarship for reasons related to athletic performance. All schools in the Power Five conference (ACC, Big Ten, Big 12, PAC-12, and SEC, and Notre Dame) abide by this rule, and some other universities have voluntarily adopted it. Here's an article explaining how the Power Five rule works:
https://informedathlete.com/the-facts-about-guaranteed-multi-year-ncaa-di-scholarships/
At schools without this or a similar agreement in place, coaches can reduce or non-renew scholarships at the end of a given year, so it is very important to know which colleges are in which category when evaluating them during the recruiting process. I know several kids who played or play in the ACC and Big 10 who have benefited from this rule. Kids who were injury prone or did not end up fitting the coach's system had no reduction to their scholarships even when they spent most of their career on the bench. Coaches certainly have tried to persuade some of these kids to quit the team to free up dollars, but so long as the athletes try to rehab from any injuries and attend training and team meetings if they are physically able, then their athletic money is secure.
And yes, it's very common to "back end" scholarships, and the percentages are put in writing as part of the NLI along with an estimate of the total value of the scholarship (at least that's how it is done with the ones I've seen). I know several kids who have 100% for their final year or two, but less for year 1 and 2. It makes sense for coaches to do it that way given the odds that a kid will quit or (in some sports) turn pro prior to their senior years.