Hello BP fam!

I have a SF rental and my tenant has 3 German Shepherd dogs that are full grown and contained in their 4' fenced yard. The neighboring property got a pittbull puppy. I also own a pittbull, so I am familiar with the breed, but I live in the burbs. 

Milwaukee Neighborhood services says that the 4' fence is enough to contain the pittbull, however, it is a puppy and will grow to easily be able to jump this soon. It likes to run and play at the fence, antagonizing my tenant's dogs, who are displaying aggressive behavior at the puppy through this chain link fence. I spoke with the neighbor and they said they can't afford a higher fence. I do not want any person or animal to get hurt, I think I'd be liable as insurer on the property if an incident happened on my lot.

Do you know:

1) What if neighbor pitty jumps fence onto my property- am I liable if this animal gets onto my property and over the fence?

2) What if a GS dog jumps over the fence into the neighbor's yard... am I liable?

3) Should I bite the bullet and put up a 6' fence to avoid any issue (don't have funds for this expense)

4) Should I call my insurance guy or would that be the worst thing to do?

Thanks in advance for your comments. 

**Please no judging on my choice to allow 3 dogs, tenant was in place turnkey and pays on time, cares for the property well.