When making reservations in restaurants, we say table for 4. is it correct also to say table of 4

No. "A table for 4" describes the table - you ask the Maitre d' for a table for 4 because your group of 4 people would like a table that is sized appropriately for them all to sit together. So you are reserving a particular size of table.

"A table of 4" describes the size of a group of people. This is not what you usually ask a maitre d' for. If you asked to reserve a table of 4 you are requesting the maitre d' to go out and find 4 people and put them at a table for you. This might be appropriate if you want to join a blackjack game - you might ask if there is a table of 4 because you want to be the fifth player.

See the difference?

Might be simpler to say a table for 4 is a table that seats only 4 people.

Table of 4 is 4 people at a table (of any size) implying the table may be large enough to seat more people but its only partly filled.

I'm sure making a reservation they will know what you want, taking it to the extreme if you reserved a table OF 4, they could seat you at a table FOR 8 and let anyone sit in the other seats.

when making reservations in restaurants, we say table for 4. is it correct also to say table of 4
Table for four is the correct way, well table for four please

A Party of 4 will need a table for 4.