What is required clearance from mechanical exhaust to garage door?
PVC isn't actually a proper material for exhaust, despite what the manufacturer says -- use stainless instead

The exhaust gas temperature of a high-efficiency condensing tankless heater operating at 140&deg;F water thermostat will hover around 170&deg;F out of the box, and this simply gets worse with time and scale buildup, reaching 300&deg;F or more as scale is a much poorer heat conductor than copper is.  This causes a PVC vent flue to "cook" (it will turn tan, then yellow, then purple/brown) and sag (PVC pipe is rated to 140&deg;F), leading to leakage and eventual total failure (in a decade or less) with its attendant CO(!!!!!!!!), hypoxia, and moisture-buildup hazards.  In fact, failed PVC venting (due to heat) on a condensing boiler system is suspected in the CO death of a family of four on a high-end rental property in Aspen.

Instead, a stainless steel concentric or double wall vent system listed to UL 1738 should be used; stainless steel vent systems, applied according to their listing instructions, will neither corrode nor fail due to heat.  If that is absolutely not an option, then high-temperature polypropylene venting (rated to 230&deg;F) can be used as a fallback, but this requires an external, engineered interlock to shut down and lock out the burner fuel supply if flue gas temperatures become excessive for the material used, as tankless heaters aren't smart enough to do that on their own (yet) and will continue to turn up the fire as they scale up in an effort to keep up with demand.

Whatever material you use, keep it clear of the garage door

The 4' horizontal or 1' vertical clearance you mention, though, should be applied to a garage door, as per IRC M1804.2.6 point 2, which applies to any "door, window, or gravity air inlet" -- in other words, any opening where natural circulation of significant quantities of air is expected when the opening is open.