Two men have already been charged after the attack on three Filipino ship workers in Avonmouth.
Police said officers have recorded the attack on three men on July 31 as a hate crime.
Three of the four are arrested on suspicion of attempted murder, with one man seriously hurt.
Police say 18-year-old Romeo Dumbi had an "unhealthy interest in serious violence".
Flames spread "quite rapidly" and "heavily damaged" the building's roof, a manager says.
Avon Fire and Rescue Service is tackling the blaze on the edge of Bristol.
The workers were killed in 2020 when a silo exploded at the firm's site in Avonmouth.
Costing National Highways more than £40m, the roundabout junction above the M49 was built in 2020.
The motorway was closed in both directions near Avonmouth, causing significant delays.
