I've done everything from 4 in an inside to 2 in a suite and it all comes down to what you can afford. If spending the 300 on the room will short you on your ability to enjoy the cruise, then it's an obvious choice.
You may want to pile all four of you into one small room (200 sq ft at the most) at home and see what you think. Bathroom area is larger in the "JS" too but again, it comes down to your needs and your budget.
Check the RC website for square footage difference between the JS and the D1 Oceanview w/ balcony. The size differs according to what ship you are on. My first recollection is that you seem to have to climb over everything in a OV, while you seem to have room to walk around in a JS. Bathrooms in most JS include a tub, while the OV has a shower with that pesky butt-grabbing shower curtain. After my wife cruised in a JS, there was no going back to OV. JMHO.
Just cruised in a JS on Rhapsody. With husband and one child, I could not imagine having a smaller cabin. The extra money for a JS was worth it to me. Also, There was a seperate, shorter embarkation line for those in a suite. And we were the first off the ship(except for those catching planes)