Can a 4 star servant become as powerful as a 5 star servant?

Yes he can, he and lancelot saber is 5 star in disguise

He hits harder than my Merlin at least. At lv 90 I prefer him over Cu Alter. Highly recommended!

If strictly in terms of raw stats, a 5* of the same level will almost always have higher max stats.

But in terms of performance, yes. As mentioned above, Herc and Saberlot are two of the few 4* who can perform on the level of a 5*, even before being grailed. Therefore grailing them boosts their already high level of performance. Some welfare 4*s like Ridertoki and Chloe are the same, even disregarding their easy access to NP5.

I've always considered Saber Alter as a SSR, she's the best saber aoe after all.

Heracles Lv90 max fou ATK: 12763
Vlad III Lv90 max fou ATK: 12489
Raikou Lv90 max fou ATK: 12546

He also has a 31% ATK boost for 3/5 turns. And Madness Enhancement B which boosts his Buster cards by 8% (he has 4, including NP). And Divinity A which boosts damage by 200. So yes, he can rival the 5* servants. In his niche of last man standing, only Cu Alter is his near equal.
Another 4* capable of rivaling the 5* is Lancelot Saber, especially when grailed to Lv90.

Some 4 star are better than 5 star servant (even at level 80) due to the fact that they have unique niche, Saberlot is an example(only art critical saber on NA).
Sadly Herc perform slightly worse (not always) than Cu Alter in his niche even at level 90 because Herc is forced to use his bond C.E. while Cu alter isn't.
PS I often use him over cu alter due to party cost and preference.

Herc with his Bond CE is the ultimate solo and last stand servant. I don't think anyone really surpasses him. He can buster brave chain for 6-8 turns even against strong bosses, which is just absurd.

Even Cu-Alter can't compete. Cu-Alter is better in a party though, because he's got better damage and protection from arrows is much more reliable when there are 3 targets.