First of all, you can't even use a grail until someone is at fully-ascended max level. You'll have plenty of time to figure out if someone is worth it.
Second, one grail is only good for two levels. You would need five grails to get a 4* to level 90.
Third, why not go for level 100 (which even 5*s will need five grails for)?
This isn't quite accurate. When Grail ascending, keep the following points in mind.
1. Grail ascending will raise the max level of a Servant by 5 at a time up to level 90, at which point it will raise the level by two at a time. An exception is made for one star Servants, which will jump directly from 60 to 70 and then +5 levels at a time thereafter.
2. Grail Ascension levels work differently than the level curve of normal levels. Different Servants will experience stat growth in differing ways, but past their usual max level all stat growth is linear.
3. Stat growth is also typically greater past the limit than usual, this effect is more pronounced the fewer stars a Servant has.
Basically the point of grail ascension is to make your Servants with fewer stars more useful at very high level play. Or if you're a whale and/or have EX rank luck you can power up your 5 stars to even higher levels.
