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.
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Three of the men arrested, aged 23, 27 and 37, are being held in custody while the investigation continues.