Halo 4 takes place four years after the end of Halo 3, and during this time period, a lot of wartime weapons and vehicles have been replaced with newer models. This is why Halo 4 looks so different to Halo 3. The issue is that for most of Halo 4's story, you are stranded on an alien planet by yourself, having crash landed on a ship that has been drifting in space for four years. The problem then, is that basically all of the weapons and vehicles that you come across, are the post-war variants, rather than the wartime stuff that should have been on the ship you crashed on. From a gameplay perspective, this is negligable: The Battle Riffle is still the Battle Rifle, it just looks rather different. Its just anachronistic, and it bothers me slightly every time I play the game.
In contrast, I just finished playing Gears of War 4 and i'm playing Gears 5 at the moment. The new enemy faction originates from wartime burial grounds from 25 years prior, and have since been untouched. Similarly to Halo 4, a lot of new models of existing weapons and equipment have been introduced, so it would have been super weird for the new enemies to appear with brand-new equipment, but lo and behold, the developers modeled in older Lancers and older grenades, just to show that they are using older equipment. Both the old and new stuff function identically to each other, and ammo is interchangeable, it is just a nice little detail.