Minimum connecting time T3 -> T4 at LHR is 90 minutes. The two flights you mention only allow 85 minutes, and therefore, don't meet this requirement.
If you'll be traveling after March 27th, or so, the TXL flight will depart T5, for which I can't yet find the Minimum Connecting Times, but I suspect you'll be out of luck there for the same reasons.
By the way, why would you willingly choose LHR? HEL is a very nice place to connect, and AY a very nice airline.