I second the notion to put an "extend a stay" in, great little device that I use all the time. I run a hot water unit and also an external grill off mine, plus it allows external fuel from tanks to be pumped inside. A royal PITA to plumb in later. I think your plan is a good one. Not sure why they did not use a T. on the inside, but doing any kind of repairs in a cramped space or simply assembly may be a reason.

Troy

<< am plumbing in my new propane tank and decided to pull the copper
tubing as well and replace it.

In the future I'm planning to add a Propex heater (not sure where yet)
and possibly a second propane tank.

So, I'm thinking that I should put in a 4-way now and cap unused ways so
I don't have to break these lines again in the future>>