Standard 2.54/1.27mm pinheader impedance
You are overthinking this issue. For 4 MHz you shouldn't have any concerns. For example, all PC mainboards use 2x5 headers for on-board USB 2.0 connections to front panel USB, and they operate at 240 MHz quite happily, on billions of PC. Although the 0.100" headers were never intended for RF interconnect and therefore are rarely characterized electrically.

Trace thickness does have a minuscule effect on trace impedance, which can be easily determined using any on-line Java microstrip calculators. Most calculators assume standard 1.5 Oz copper thickness, which always fall within the margins of error.