UPDATE 4-Australia's Transurban spurns $4.4 bln buyout bid

* Transurban leaves door open to takeover talks

* Shares surge 20 percent to 9-month high

* Canadian funds offered A$5.25 a share

* Canadian funds want maximum 78 pct stake in unlisted group

* Latest Canadian swoop on battered infrastructure groups (Adds offer details paras 6,9,10)

By Sonali Paul

MELBOURNE, Nov 5 (Reuters) - Australian toll road operator Transurban Group (TCL.AX) rejected a $4.4 billion buyout approach from two Canadian pension funds on Thursday, but left the door open to a better offer, sending its shares up 20 percent.

The offer, pitched 20 percent above its last close, marks the latest swoop on an Australian infrastructure group by Canada's pension funds, taking advantage of beaten-down share prices as their targets struggle to refinance debt in the credit crunch.

The offer for the owner of roads in Sydney, Melbourne and Virginia in the United States came from the Canada Pension Plan Investment Board and the Ontario Teachers' Pension Plan Board, which together already own about 28 percent of the group.