I-485 in most of Charlotte - Western segment to I-85, southern segment, and the eastern segment. Was built with 4 lanes but only widened to 6 lanes and express lanes there are currently under construction.
I-540 in Northern Raleigh. Should have been 8 lanes, urban sprawl kicked in and it gets very congested. Ramp meters did help, but it still needs work.
I-40 from Hillsborough to Morrisville. Was built with 4 lanes, although widened to 6 lanes in 2004. Still gets congested and more upgrades are needed on it. It was built with a 46 foot median so widening it would be very expensive.
I have a feeling I-295 in Fayetteville might jump on this list because it was built entirely with 4 lanes.
I-40 bypass in Winston-Salem. Opened in 1992, has 4 lanes, although planned to be widened to 6, I say it needs 8 to prevent more future widening!
I-785 in Greensboro. Opened in 2002, has 4 lanes, and when the I-840 loop is finished, traffic volumes on that segment will be expected to increase quickly, warranting the need to widen it to 6 lanes.
I-40 in Western Raleigh, Eastern Cary. Has 4 lanes, widened to 6, although they left space for a future lane (either 8 GP lanes or they plan to add a toll lane in each direction).