I'm trying to find adventures that are meant for 4 players and I haven't been able to find any. I've found adventures for either 3 players or for 5.
Are there any adventures that were ever made for 4?
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As noted in a Keep on the Shadowfell answer by one of our resident 4e experts, all official modules for 4e (and consequently most fan-built modules) are intended for a party of 5. This other answer lists lists of 3rd party sources. I haven't gone through them all, but going through them is nonetheless your best bet if you are, for some reason, dead set against modifying a 5 person module yourself, which, short of drafting your own campaign, is probably the best option.
If I remember correctly, all adventures were optimized for 5 players, but scaling it down to 4 should not be a problem at all. The 4e DMG gives you lots of suggestions for doing that. And if you're looking for something to run in 4e, my absolute favorite was The Madness at Gardmore Abbey. I actually ended up running it twice with different groups.
All Living Forgotten Realms modules have adjustments for playing with 4 players (as well as adjustments for playing with 6 players). See the Living Forgotten Realms site.