What are these for?

Rebuilding my Z1B carbs and not sure what these plastic tubes are for. Do I have the fuel lines on right?
 

Those are your Vent tees for the float bowls. You have two, 1&2 share one and 3&4 share one. Don't plug them or you'll have all kinds of weird happenings. 

do I need to put hoses on them or just leave them like that?
This diagram shows hoses, but I also see hoses on the float bowl drains. That's a lot of hoses 
 

Hoses are a good idea. If you have a float valve failure, the hose will dump the extra fuel on the ground behind the engine rather than all over the top of the transmission case. The hose will also make it much less likely for dirt and bugs to get into the float bowl. Also also, the fumes are less likely to get into your nose when vented behind and below the engine.

Put short lengths of tubing on them and hang them down between the carbs or behind the engine.
With no tubes on the upward facing open end of the brass tubes will allow dust, debris and water etc to get in.

The carb bowls need access to atmospheric pressure.  That's what the tube provides.  I put hoses on the two tubes and extend them upward and back.  There is an oval shaped hole in the bracket where your rubber strap for the tank is located.  I tuck the two hoses through that hole.