I wouldn't say "usually", but it's certainly often. And in some cases, disparate DIMMs will simply not play nice together. Other times, it will work just fine. It really matters whether the board likes the stick at all and whether the DIMMs will play nice together. If their specs including the timings, speed and voltage are too different, they just won't no matter what. Other times, as you say, they will but everything will just default to whatever the lowest JEDEC values are that can work for both sticks.
And knowing what the board is is helpful because there may not even BE a slot 4 if the board has only two DIMM slots. The majority of microATX boards and even some ATX overclocking boards, and certainly most ITX boards, have only two. Knowing facts is how we can make determinations. Without them, it's all just a guess.