Does Alien Swarm tune difficulty when play with less than 4 marines?

The answer to your question would appear to be, in fact, yes. I too have looked at the game's source code, most notably in game\shared\swarm\asw_gamerules.cpp and this is what I've found.

There is a number that governs difficulty. In the source it's called m_iMissionDifficulty, though the function GetMissionDifficulty() is often used to read its value. Its initial value is set using the skill level you choose.

• Easy: 3
• Normal: 5
• Hard: 7
• Insane: 10
• Brutal: 13

If there are three marines playing, this number is reduced by one. If there are one or two marines playing, this number is reduced by two. The number can not fall below two (i.e. if you're playing using the easy skill level).

This number's practical application is in various calculations used to determine damage dealt.

This tuning can be toggled using the cvar asw_adjust_difficulty_by_number_of_marines - on by default - though sv_cheats must be enabled in order to disable it.

I doubt it's going to change the intermediate difficulty, so how hard Normal is for example, but you can always vote for a lower difficulty by clicking the button next to the chat button, or in the mission details screen.

Not in my experience.

Yesterday I played only with a friend and the amount of aliens was the same as with four (I distinctly remembered a few parts of the level as I had just previously played it with four), not to mention that we ended up dead a lot more times.