With regards to Primera, I'm not sure it would even be possible to fly to the Canaries (or even Malaga).
The Newark flight arrives into BHX at 10:30am and leaves for Newark at 17:45, giving the aircraft 7hrs 15mins. A return trip to Tenerife takes between 9.5 and 10 hours. Similarly, Malaga is approximately a 6.5hr round trip (plus 2 turnarounds at BHX - EWR-AGP & AGP-EWR).
On the other hand, a return trip to Stockholm takes 5.5 hours - so possible.
As to whether or not any potential 4th destination is long haul, I'm not sure how that can be done. 1 aircraft will be flying to Newark daily, whilst the other will be doing 4 flights per week to Boston. If Toronto is the third destination and is served 3 times per week, another aircraft would be needed to serve a 4th long haul destination (unless they made Toronto a seasonal summer destination with the 4th destination being seasonal winter).
But with Monarch now gone they might see an opportunity at BHX and get there hands on some leased aircraft ? Maybe.