They sell you the same old engine  in COD4 and 5 to you.

All three Call of Duty games use Radiant, the map design tool used for every one of id's engines and games that use the engine. Q3Radiant from COD1 to the in-name-only CODRadiant in COD2 and COD4, has been almost unchanged, only added upon.

The very weapon files, little more than text without a file extension, have not changed at all, only had a few new variables added to them.
In COD1, they had "fire selectors" for a couple weapons like the Thompson and BAR. That selectior was a "weapon" selector. There were two Thompson weapons, one that fired in single shots, the other automatically, the fire selector key switched between the two versions.
In COD4, those "attachments" are not actually one weapon with different stuff on it, but different weapons on their own that shared ammo with one another. There was no "Fire Selector" in COD4, but the Grenade Launcher, also a seperate weapon entity from the version that fires bullets, is literally the same thing.

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Call of Duty 1, using the licensed Quake 3 engine, had .tga and even .jpg files for textures. They also used the .pk3 archive file (which was literally a .zip with a different extension).

After Quake 3 went open source, Call of Duty 2 took the engine and made it "their own" - improving the graphics, adopting some stuff from Doom 3 such as .dds files and making minor changes so that "their engine" was further seperated from the source of Q3. PK3 files were renamed .iwd for little more than a technicality; it was still the same exact .zip file with a different suffix.

Aside from graphics, the core engine is unchanged, same quirks (such as breaking the speed barrier by using momentum and movement in an adjactent direction), limitations (model and entity limits) and flaws of Quake 3, data and scripts carried over from COD1, same exact gameplay of Call of Duty 1 (though with much better design ideas and truly accurate weapons).

Call of Duty 4 has the same .pk3 .zip .iwd files. For whatever reason, .dds files were renamed .iwi and had a single byte on the header changed. Many things were added in this iteration, Perks, Hardpoints, Penetration, another boost in graphics with HDR effects, but the existing game engine remained very much the same.

If it was in COD1, it is also in COD4 and has barely (if not at all) changed.

Heck, there were a couple left-over graphics in Call of Duty 2's files from COD1 and leftovers from COD2 in COD4.
Just about every variable you could access through console in COD1 is still in COD4.

CoD4's engine is a heavily rewritten/customized CoD engine, which is in turn a proprietary license of ID Tech 3 (Q3A) which John Carmack based off the design of the Q2 and Q1 engines.

OH NO GUISE WE ARE STUCK IN 1995 CALL THE INTERNET AMBULANCE!@#

the strongest point of COD4 is not its graphics. it is the extremely fun and exciting gameplay. it is superb...it is awesome...it is from infinity wards. (need some voice from the preview guy...coming this summer...)

perks? what the hell do perks have to do with cod MW 2?

in cod4, there isn't anything about perks wo ??!?

the strongest point of COD4 is not its graphics. it is the extremely fun and exciting gameplay. it is superb...it is awesome...it is from infinity wards. (need some voice from the preview guy...coming this summer...)