Investigators said that they had ruled out a xenophobic motive for the attack in Villingen-Schwenningen, Baden-Württemberg, which they believe was directed at the security guards at the home by their competitors.
The grenade was thrown at the container used by the security guards in late January – but failed to explode.
No-one was hurt in the attack, but police said that it was a live grenade.
"Conflicts that exist between the security services active in the Schwarzwald-Baar district could be the cause," a police statement read.
SWAT (SEK) teams were brought in to help beat officers arrest the four men for suspected breaches of weapons control laws.
Three of the men arrested, aged 23, 27 and 37, are being held in custody while the investigation continues.