Yip, the 109K4 will hold the cards in almost everything while it has WEP. But given that the 109s have twice the WEP endurance than the 51, its more likely you'll be dead before the 109's WEP runs out.
Without WEP the 109 looses a bit acceleration and speed, but compared to the pony without WEP, it still climbs and accelerates better (there is a spot about 10k where they're pretty close). The pony could disengage (aka run away) as its faster with the 109 out of WEP. But the 51 will run out of WEP first more than likely and once it does it will still have to spend at least 5 mins dodging the 109 at full noise till it runs out, so its likely you'll be mince before then anyway - given equal pilots and equal E stakes initially.
Sustained turn goes to the K4. Very high speed control effectiveness goes the ponies way, but in a on the deck knife fight that aint much help.
Since the K4 has vastly better acceleration than the 51 and a better top speed on WEP, the pony really needs an excess of energy to even try to dominate the K4. As it will not be able to replenish lost E as fast as the K4, its window of opportunity will be limited. Turn up with a excess of E, use a bit of it to try get the K4 while you're hot; and while you still have enough E to escape, it may be wise to do so if you want to fly home.
So given equal pilots and an equal E footing to start with and assuming the K4 pilot doesn't do something silly or fall for a cheap trick, the pony really can't hope to win.
But you never know the K4 may be piloted by a chump or even a good one who made a mistake which you can capitalise on. Fortune favours the bold.