Amsterdam to Paris by train. Can we stop in Brussels for 4 or 5 hours for a quick tour of

Yeah. You would just have to buy 2 separate tickets on the Belgian rail site.
Amsterdam to Brussels Central
Brussels Central to Paris.
The Grand Place is very close to the Brussels Central train station.

The Brussels Nord train station has bag storage. When are you planning tp do this so we know how soon you should buy tickets? They usually go on sale 3 to 4 months on advance

You can also take the fast train (Thalys, Oui.SNCF) to Brussels, do your thing, then when ready just get a ticket that day to Amsterdam on an Intercity train, would take a little longer but not terribly. Trains are frequent, so you would not be rushing to get back to the station to catch a specific train.

Also note that the fast trains from Paris stop at Brussels Midi/Zuid, most of the sights you want are closer to Brussels Central. Your Thalys ticket is valid on the train connection to Central.

Yeah Brussels Midi has bag storage.

Paul's response works if you're headed northbound, since there are cheaper, unreserved train options between Brussels and Amsterdam. It's not the same headed southbound since the later train in that case is the reserved Thalys train to Paris, which is expensive to buy on short notice and theoretically also can fill up, so you likely want to book ahead.

Be sure to have lots of coins for the bag storage at the train station (that was the big struggle for us!). Agreed that 4-5 hours is too long.

Yes, you want to visit the Grand Palace. From Aug 13-16, this square will be filled with a giant flower carpet that would be worth seeing. So is the Royal Museums of Fine Arts of Belgium.

To be honest I wouldn't bother. It seems to me from this site that people who try to do Brussels in a day or less tend to be disappointed. The really interesting and attractive spots are dotted around over quite a wide area.