Everyone loves a 20 main-stat but the difference between 20 and 18 is only 5% greater success so if a feat is important enough an 18 might be good enough. (8th Level is very far away in playing time so what you choose at 4th level needs to be impactful)
Elemental Adept can be exceedingly important if your mage is focusing on a single element.(Draconic Sorcerers)
Resilient Constitution is important for Wizards and Clerics and Druids and Warlocks.
War Caster is great for all spell-casters.
Alert is fantastic for non-evocation mages.
Crossbow Expert, if it applies to spells as it does in 5e, is great for Warlocks.
Feats are certainly powerful, but I think you're underestimating the impact of those 5%. If you're attacking an AC 14 target with your 18 Str and a +7 attack bonus, you have a 30% chance to miss. Increasing Str to 20 will make that 25% - that's a roughly 17% reduction in the total amount of attacks you miss. This is very important for spellcasters with lots of save-or-suck spells and even more so for Dex characters where that one stat affects their attacks, AC, initiative and a common save. The higher a stat is, the more valuable that one extra point becomes, especially with AC.
The feats you mention are absolutely worth taking over a stat increase though. I'm just saying I think getting that +1 can be a comparably strong choice too, if perhaps a boring one.
