The 45-year-old man was approached on Castle Street in Dudley at around 1am on Wednesday by four youths who demanded he hand over his bag.
He was then kicked and punched on the floor and had a glass smashed over his head after he refused, before the group ran off as people came to intervene.
West Midlands Ambulance Service were called to the scene and the man was treated for injuries to his face and foot and subsequently taken to hospital to received further treatment.
Police have since arrested a 15-year-old boy, 15-year-old girl, a 14-year-old girl and a 12-year-old girl on arrested on suspicion of assault. They remained in police custody for questioning on Wednesday afternoon.
A spokesman for West Midlands Police said: "Four people have been arrested after a man was assaulted in Dudley this morning (6 July).
"At around 1am the 45-year-old was approached by four people who demanded that he handover his bag.
"When he refused they kicked and punched him on the floor. A glass was also smashed over his head.
"People who saw what was happening intervened and the group ran off.
"We were called by our colleagues at West Midlands Ambulance Service. With the suspects believed to still be in the area, an area search was carried out and CCTV was reviewed.
"Minutes later, a 15-year-old boy, 15-year-old girl, a 14-year-old girl and a 12-year-old girl were arrested on suspicion of assault and remain in police custody for questioning.
"The man was taken to hospital to receive treatment for injuries to his face and foot.
"A big thank you to those people intervened and helped the man."
A spokesman for West Midlands Ambulance Service said: "We were called to reports of an assault on Castle Street at 12.54pm and informed police colleagues.
"Police officers were first on scene and stood us down as we were no longer required."