Why are Puerto Rico, Guam, Northern Mariana Islands and the Virgin Islands not yet American states?

They don't want to be states.

The people don't want to change to statehood. Residents of US territories are US citizens with all the rights of someone who lives in a US state except they can't vote in federal elections. In exchange, they pay no federal taxes. 

The U.S. has always allowed territories to APPLY for statehood

at which point Congress evaluates and votes whether or not the territory is to become a state.

None of these have applied for statehood.

Guam applied for Commonwealth status (i.e. same status as Puerto Rico), but the Commonwealth proposed by Guam did not meet U.S. legal standards and so was rejected.

Northern Mariana Islands, like Puerto Rico, is a Commonwealth. Like Puerto Rico it has not applied for statehood.

The U.S. Virgin Islands is not a Commonwealth and has not applied for statehood.

>> Would making them states cost more or less money to union? <<

Almost certainly more, in the form of federal support for things like education and infrastructure. However: in exchange for less self-governance. IMO the politicians successfully sway their people on the grounds that it's better to self-govern than to enjoy Federal funding with just one voting Representative in the House (but two in the Senate).

Because they don't WANT to be states. I lived in Puerto Rico nearly 50 years ago. There was a push back then for the commonwealth to become a state, but they couldn't get a majority of citizens to vote for it. It's still the same.

Why buy the cow when you can get the milk for free? Puerto Ricans have all the benefits of being American citizens, with none of the responsibilities... I would imagine it's the same for Guam, The Marianas and the Virgin Islands.