This is a bit complicated.
To be honest yes you should "down size" your adr........BUT that would mean cancelling your current adr and redoing them.
So the hassle is do the restaurants have open adrs for your times? If not then that means you won't be able to get a new ressie for the correct # of people.
Meaning you could lose your adrs. What I would do is this, go online and try to book the restaurant for the time you want with the correct number of people. If you can then reserve that and then cancel the original one.
Now if there is no other available adr and you really want to eat there, just hold it and when you show up at the restaurant let them know at the podeum that it is only 4 not 6.
You will get different answers to this question, yes if you know you don't need that many seats you are keeping others from being able to book them, but on the down side you don't want to lose what you have.
Good luck!