Not to be an ass, but why do you care? You're not getting 100Mbps over a leased line unless you're talking OC3 or better, and at that point, is 5mbps really going to make or break the usability of your connection? But since you asked, here's a couple for you:
OC1: 51.84 Mbit/s (payload: 50.112 Mbit/s; overhead: 1.728 Mbit/s)
OC3: 155.52 Mbit/s (payload: 150.336 Mbit/s; overhead: 5.184 Mbit/s)
OC12: 621.84 Mbit/s (payload: 601.344 Mbit/s; overhead: 20.736 Mbit/s)
OC48: 2488.32 Mbit/s (payload: 2405.376 Mbit/s; overhead: 82.944 Mbit/s)
OC192: 9953.28 Mbit/s (payload: 9621.504 Mbit/s; overhead: 331.776 Mbit/s)
So that's about 3% for SONET, I don't have any hard numbers for T-carrier lines, but it should be right around 8% once you add HDLC encapsulation. Your theoretical 4Mbps line can actually push around 3.7Mbps in each direction. Which shouldn't make a marked difference unless you're pushing so much data that it'd be faster to drive there with a data tape and run a restore. In which case, do that.