Looking at below image, which compares heights of rockets, I couldn’t help but notice that the Ariane 5 was a short and fat rocket compared to the Ariane 4 (tall and thin). Why would this be advantageous for the Ariane 5?
Ariane 5 core burns LOX and Liquid Hydrogen. This needs larger tanks and thus is wider.
Ariane 4 uses hypergolics that are much denser. (Nitrogen Tetroxide, UMDH). Thus the tanks are smaller.
(Consider Delta IV using LOX/LH at 5 meters width vs the Atlas V using Kerosene and LOX narrower around 3m. But about similar performance ranges).
Because the Ariane 5 boosters are considerable bigger. The weight of the Ariane 4 ranges from 240-320 tonnes. The weight of Ariane 5's boosters is 270 tonnes each for a total of 540 tonnes. The total weight being 850 tonnes, this is a large portion. Therefore, Ariane 5 isn't as tall because, well, it has the majority of its mass stored on the side in the boosters. On Ariane 4 the boosters never take more than a 1/5 of the mass. Also Ariane 4 had launch configurations with out boosters, Ariane 5 would never lift off the ground without them.