If you can get the overall length of the vice, then halve it. That should then roughly be the width of your table. So if the 4″ vice is 12″ (300mm) overall length, then that will be the largest you should fit to your table, which is 6″ (150mm) wide. Half on, half off, overhanging the front of the table.
Be VERY careful, some cast vices look to be very good value, but when you open the jaws, where there should be support next to the fixed jaw for resting your parallels on, you can find it is just a rough cast trough. The last one on the page in the link in David’s post above is just such a fellow, I should know, I am having to fill in the crappy area and refinish it on the surface grinder for a friend.
I have been using Vertex ‘K’ series for many years, and have yet to buy a bad one. The only problem is with them is that the support under the jaws doesn’t go the full width of the jaws.
The ‘Kurt’ type clones seem very popular as they usually open that little bit wider, and the holding area at the bottom of the jaws usually goes right across the full width of the jaws, but again, there are some rough ones about, plus also there is no centre support on them, and tiny bits can fall under the vice on some, past the main leadscrew.
You can see what I mean from the pictures about no centre support, and the leadscrew running thru the centre.
I hope this helps.
Bogs