Adding a related FSC article for anyone working on OBS / GMT400 headlight projects: the new OBS / GMT400 4-Hi Mod guide walks through the relay-based wiring that keeps the low beams on when you switch to high so all four bulbs run together.
Covers parts, the install procedure, the testing checklist, and the DRL backfeed gotcha that catches a lot of 1990+ Canadian and 1995+ US trucks.
Discussion thread for results, wiring methods, and troubleshooting is here: OBS / GMT400 4-Hi Headlight Mod — Wiring Methods, Results and Troubleshooting