[size=3]Table 310.15(B)(6) Conductor Types and Sizes for 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders.
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[size=3]Conductor Types RHH, RHW, RHW-2, THHN, THHW, THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE, USE, USE-2
Question: Does this table in the 2008 NEC permit a 4 AWG Copper to be used for 100 ampere service entrance conductors if the insulation is over 40 years old?
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That is always a good question. Does a U/L listing expire when newer products come along? If I found a roll of old 60c asphalt/paper jacket NM in my basement, could I use it in new construction?
For both Joe and Greg
Joe
If the conductor type matches the table list, I would say yes.
Greg
The older cable you mention does not have conductors rated at 90C, so you have to “sneak” it in and just say it is an “old” installation. 
The Table mentioned was not in the NEC when some older installations of 4 AWG were installed, and they were probably Type R, my point was that we cannot make new rules fit older installations, and that an unidentified Type wire sized at 4 AWG is not rated for 100 A in accccordance with Table 310-12 in earlier codes.