Normally, I recommend buying Gildan G640 white t-shirts. These are the sorts of t-shirts that get used for silk screening and the like, so they're a 4.5oz., 100% ringspun, pre-shrunk cotton. I've used these shirts as undershirts, and they wear like iron. Unfortunately, I'm at a loss on the sizing as they don't offer a 4XLT, only a 4XL, and I can't even find 4XL sizing measurements in any catalogues to see if you can fudge it. This said, I'm technically an XLT, and their XL has always been long enough to keep tucked. YMMV. Unfortunately, given the size in question, what normally costs under $3/shirt will run over $8. It's also worth noting that if he likes a "tighter" undershirt fit, buying the next size down
sometimes works well... after all, they're cut as t-shirts, not undershirts, so they might be a bit baggy and the sleeves might be a hair too long with some short-sleeve shirts. Don't just do, though, check sizing.
If you want to explore the options, give
Jiffy Shirts a look (it's a direct link to the G640). They're the place I've bought from in the past, they have good prices, and decent customer support... but it's also been a while since I last bought any. I also looked to see if they even had 4XLT from any manufacturer, but they only listed 3XLT as their largest "tall" size.