Unforturately, you do not have a leasehold entitlement to the property since you are not paying rent under an oral or written agreement.  You are therefore a squatter.  As such, you may be evicted by the "owner" of the property.  While your girlfriend is not the owner, i think that at the end of the day, she can simply have her dad ask you to leave or serve you with papers to leave.  I suggest asking for an extra week to locate a new place to live as it would take them over a month to evict you going thru the court procedures, which you can say you will force them to do if you do not get this time.  You have been there for 4 years and i assume you are a good dad.   She should not unreasonably withhold this request - make sure the dad is apart of this meeting so you are all on the same page and put something in writing saying that you can stay untill a certain date so everyone knows that there will be finality.