I'd heard talk that No. 4 needed a new smokebox, which I suppose could be true since the loco's smokebox was clearly removed. I can't imagine why you'd remove it if it could be rebuilt or repaired in place.
No. 4 never had a superheater, she's a saturated steam locomotive. Lots of wet steam over the years probably caused her smokebox to corrode from the inside out. She supposedly earned the nickname “Old Slobberface” because of the condensation that constantly dripped from her (leaking) smokebox front.
I've also heard No. 4 needed a good deal of firebox and boiler work. Supposedly, the NCTM started installing new firebox sheets, but never finished the job. Rumors say she's also suffered a lot of boiler corrosion. Ideally, steam locomotives should use treated water, to avoid boiler scaling, corrosion, foaming, etc. Typically, the Class I railroads built water treatment plants at the water stations located along their routes, something the BC&G and the tourist lines where No. 4 ran may or may not have had in place.
The photo below shows No. 4's running gear after being trucked in from N.C. to the Cass locomotive shop. The shop has a drop pit, installed when the shop was built back in the 70s, but since No. 4 is the first rod locomotive to be refurbished at the Cass shop, this will be the first time the drop pit will be put to use!
Yeah, poor No. 7. She only worked a few years at Cass before developing major boiler problems. Cass bought a 70-ton Shay (Graham County RR #1926) "for parts" a few years ago, but I've never heard if its boiler was repairable or not. If it is, maybe it could be used as a donor boiler for No. 7 sometime in the future. But then Cass has (I think) ten geared locomotives in operation now, plus D&GV's Climax, so I guess they don't have a real motive power shortage at the present time.
But they die operate five at once during the Cass Railfan Weekend just a few weeks ago, as one triple-header and one double-header on double-tracks, which you can watch in the video below starting at about 2:55.
More videos of the 2015 Cass Railfan Weekend: Lots of run-bys, whistles galore, simulated freight trains, etc.