How do I make sure my ETH CV only gets 4 sockets when larzuk socketing???

unless the number of max sockets have changed, there is no way to get more than 4

CV has a maximum of 4 sockets, so you should always get 4 open sockets from Larzuk.

http://classic.battle.net/diablo2exp/items/elite/polearms.shtml

However the maximum socket an item can have is also based on the ilvl, which in terms is based on the area lvl it was found.

See

https://diablo2.diablowiki.net/Sockets -> for sockets per ilvl
https://diablo2.diablowiki.net/Area_Level -> for area level

A CV has always 4 maximum sockets unless it has an item level of < 26. However a CV cannot drop in an area of such low level, so any found CV has a maximum of 4 sockets.

There is a small exception and that is when using a cube recipe on an item (for example turn a low quality item in a normal quality one) it turns the ilvl to 1, and in this case you would get only 3 sockets.

While this might be a lot to take, you can be certain that any CV you found yourself will get a maximum of 4 sockets when doing the Larzuk quest. I'd say that the same holds for 99.9% of the CVs you trade