yes, you shouldnt do that, the sticks should have the same or similar specs
for the speed u want to use the same mhz or above, but theres no point on getting one with higher mhz because it will just slow down to match the other sticks speed
check if the RAM in the 6gb laptop works with the 4gb one. If they match and can be used together you should be able to.
I think you shouldnt do DDR4 with DDR3 (check this I am not 100% sure) but you are most likely fine.
If the sticks don't match just don't do it.
Alternative answer, stop being Turkish.
ddr3 wont fit in a ddr4 slot, and vice versa
pc ram wont fit in laptop ram slot, and vice versa
i dont think laps works whit ddr4 :D some new one maybe