If it's having trouble starting in cold weather, and no engine lights are on, one or more glow plugs are knowingly shot, not necessarily the injectors. Smoking can be caused my numerous things, even poor fuel. Make sure the fuel/water filter separator has been changed recently. There are also some diesel fuel conditioners you can try, increase the cetane rating and so on, so shop around. The last thing I'd want to do is dump $4K on injectors. If the injectors are in fact faulty shop around local yards. Chances are good that you'll pay well under half for a used injector than a new one in just as good shape, if not better. RockAuto Auto Parts
is a good source.
I wouldn't necessarily replace all 8 as it's not necessary at this point. Even if you replace the two the dealer said were 'shot' that will also lower the bill temporarily and you can get the other two replaced in a month or two. Dealers will try their antics to replace as much as possible while the trucks in the shop. It can save ever so slightly on labour costs to replace all 8, but what's to say that one of those new injectors won't fail in 20K or 30K miles? If reliability is a main concern and you're consistently out of town, then it might be best to replace the four injectors in question, but I'd just replace the two for now and go from there.