Police in Hull believe sexual assaults committed against two women and two girls are linked.
Humberside Police said the victims, who were aged 14, 16, 21 and 22, were attacked between 17 and 19 October.
They were all approached by a man on a bicycle who indecently assaulted them before cycling away.
Detectives said they were linking the attacks and police patrols have been stepped up in the areas of the city where they took place.
A force spokesman said the first attack on a 21-year-old woman happened at about 2315 BST on 17 October near Marks and Spencers on Whitefriargate.
The second incident involving a 22-year-old woman took place at 0930 BST on 19 October on Paragon Street.
A 16-year-old girl was attacked on the same day at 1930 BST on Chanterlands Avenue.
The last assault, on a 14-year-old girl, happened on Spring Bank West at its junction with Walton Street an hour later.
Patrols have been increased in the area and offices are examining CCTV footage.
The man is described as aged between 35 and 45 and about 5ft 8ins tall.
He is balding with scruffy-looking dark brown hair and was wearing grey sweat pants, a grey hooded jacket and a blue T-shirt.
In the incidents on 19 October he was wearing a black fleece and black track suit bottoms.
Anyone with information is asked to contact Humberside Police.