You seem reluctant or unable to quantify “high”.
Ask these guys:
https://www.ecdc.europa.eu/sites/de...gement-contact-novel-coronavirus-cases-EU.pdf
“1. High-risk exposure (close contacts*)
• A person living in the same household as a COVID-19 case
• A person having had direct physical contact with a COVID-19 case (e.g. shaking hands)
• A person having unprotected direct contact with infectious secretions of a COVID-19 case (e.g. being
coughed on, touching used paper tissues with a bare hand)
• A person having had face-to-face contact with a COVID-19 case within 2 metres [2] and > 15 minutes
• A person who was in a closed environment (e.g. classroom, meeting room, hospital waiting room, etc.)
with a COVID-19 case for 15 minutes or more and at a distance of less than 2 metres
• A healthcare worker (HCW) or other person providing direct care for a COVID-19 case, or laboratory
workers handling specimens from a COVID-19 case without recommended PPE or with a possible breach
of PPE [3]
• A contact in an aircraft sitting within two seats (in any direction) of the COVID-19 case, travel companions
or persons providing care, and crew members serving in the section of the aircraft where the index case was seated [4] (if severity of symptoms or movement of the case indicate more extensive exposure, passengers seated in the entire section or all passengers on the aircraft may be considered close contacts)
2. Low-risk exposure (casual contact)
• A person who was in a closed environment with a COVID-19 case for less than 15 min or at a distance of more than 2 metres
• A person having had face-to-face contact with a COVID-19 case for less than 15 min and at a distance of less than 2 metres
• Traveling together with a COVID-19 case in any kind of conveyance.
Longer duration of contact increases the risk of transmission; the 15-minute limit is arbitrarily selected for practical purposes. Based on individual risk assessments, public health authorities may consider expanding contact tracing and management to persons who had a shorter duration of contact with a case.”
As I say, someone/anyone in the same vehicle as someone infected for any length of time (and certainly four hours), has a high risk of contracting the virus.