* Four suitors vying for UniCredit's Pioneer
* Amundi offered 4 bln euro in non-binding bid
* Poste Italiane looking for a partner to sweeten offer (Adds quotes and details)
By Francesca Landini and Gianluca Semeraro
MILAN, Oct 6 (Reuters) - France's Amundi has made a higher than expected 4 billion euro ($4.5 bln) offer to buy UniCredit's Pioneer Investments, two sources told Reuters, raising the bar in the race to snap up Italy's third-largest asset manager.
Three other suitors are vying for Pioneer - an Italian consortium led by national post office Poste Italiane, Australia's Macquarie and British group Aberdeen Asset Management, sources have said.
UniCredit's recently appointed CEO Jean Pierre Mustier is looking to sell Pioneer and other businesses - including online broker Fineco Bank and Polish lender Pekao Bank - to boost the bank's stretched capital base.
Pioneer's value had previously been estimated at around 3 billion euros.
Sources familiar with the matter told Reuters last week that Italy's biggest lender by assets may need to raise as much as 16 billion euros through asset disposals and a planned share sale. Netting 4 billion euros for Pioneer would help the bank reduce the size of the rights issue.
Amundi, Europe's biggest asset manager, and UniCredit declined to comment.
Shares in UniCredit rose nearly 3 percent on expectations Amundi's offer, which was first reported by Il Messaggero daily on Thursday, would force the other bidders to sweeten their own offers.
One source said Poste Italiane, which is leading a consortium made up of state holding firm Cassa Depositi e Prestiti and domestic asset manager Anima Holding, was now looking for an additional partner to improve its bid for Pioneer. It could also try to join forces with either Aberdeen or Macquarie, according to another source.
According to sources, Amundi, the Poste-led consortium, Macquarie and Aberdeen Asset Management have all gained access to the data room of Pioneer, which manages over 220 billion euros in assets and has three main offices located in Milan, Boston and Dublin.
Binding offers are due by Nov. 3, one source said. A final decision by UniCredit is expected after a Dec. 4 referendum on constitutional reform that could unseat Prime Minister Matteo Renzi's government, creating financial market jitters that could affect UniCredit's fund-raising efforts.
Mustier will unveil his new strategic plan on Dec. 13.
$1 = 0.8957 euros Additional reporting by Elisa Anzolin; Writing by Francesca Landini; Editing by Susan Fenton