It looks like May 4 and 5 are holidays in the Netherlands. Do you know if sights will close or if train service will be effected?

On May 4th we commemorate those who died in WWII; if you happen to be in Amsterdam, there is a ceremony on Dam Square, and a minutes silence. Nothing else.
On May 5 we celebrate the end of WWII: everything will be open, it's not a day off for people.
Everything is open as usual on these days.

Schools have May holidays from the end of April (including King's Day). Places such as amusement parks will be busy.

There are remembrance ceremonies is most towns and villages on the 4th. The two minutes silence is at 8pm, so shops and museums are usually closed by 7pm.

Civil servants and a few other lucky ones have May 5th off but the majority of people do not, and have a normal working day. It is a school holiday. Increasingly schools take two weeks off then and many people go away then rather than in the summer as it is cheaper. The official week for the May school holiday is 28 April to 6 May.

If you are on Texel on the 4th, do try to attend the Remembrance ceremony there.

If your travel in NL is during the May break, make sure to book everything early. The Wadden Islands are very popular, increasingly during the Spring holidays.

helpful info. i knew about Kings Day and am working on my schedule to avoid Amsterdam then, sounds like lots of people.

You should embrace Kings Day, whether in Amsterdam or another town/city. there will be lots of people no matter what day it is, and Kings Day is a very uniquely Dutch thing. Practically every town and village has something to mark it, some more than others.

Amsterdam is an outlier on Koningsdag, because it's so massive, but there are celebrations everywhere, as hetismij2 said. On a smaller scale, more intimate, but with the common ingredients: children selling stuff from the attic on the street, street parties, something to do for the children, bars and cafes open. Wear something orange and join in!

sounds like fun and I do like orange.