At the end of the day, a fully upgraded 4-star (and even a maxed out 3-star) is significantly better than a bare legendary Servant. Most of the time, your legendaries will be under-leveled compared to your 4-stars, which is why, as an F2P player, you might prefer the latter over the former.
Yet, even if you find your favorite 5-star, it can take ages until you pull another copy to upgrade their NP or until you gather the right mats to level them.
When fully upgraded, a 5-star Servant will always be better than a 4-star one. Fortunately, a few lesser Servants can be just as useful in terms of progression, especially since you’re more likely to find copies of them during summons. This, however, doesn’t mean that you can’t have some fun while you’re at it. As an F2P player in Fate/Grand Order, it can be extremely difficult to find and upgrade some of the best legendaries in the game.