In DMC4, you fight Berial 3 times, Echidna 3 times, The frog boss 3 times, and Agnus 3 times. You fight Sanctus 2 times, and Credo once. You fight the Savior once with Dante.
In DMC1, you fight Phantom twice, and you have 2 chase scenes with him. You fight Griffon 3 times, and Nelo Angelo 3 times. You fight Phantom 3 times. Mundus has three phases, sky battle, lava battle, and the last stand at the end.
....so why did everyone complain the repeated bosses in 4, when 1 had them too?
Because you don't fight them through the levels in nearly exact reverse order and the boss in the exact reverse order. Other action games at least have the decency to mix them up. DMC4 executed its recycled thing very very poorly.
Because even if you fight the same bosses several times in dmc1 the fights are often diffrent enough. Fighting griffon on the ship where he pins the laser bird on you is different then fighting him on land, or in the coliseum where after he loses his wing he starts barraging you with the beam paterns.
The fights have different areas and moves. In DMC4 you fight them but they are the same. Belial holds no new suprises the 2 and 3rd time you fight him, heck even the arena is the same.