It would cost an estimated $4.2 million to rebuild a Back Bay apartment building destroyed by a March 26 fire that also killed two firefighters, according to court documents. The building was assessed for $1,628,500. The owner of 298 Beacon St., the estate of Michael J. Callahan, has filed a lawsuit against a welding company, D&J Iron Works in Malden, alleging that workers were careless when installing an iron railing in the back of a neighboring building. Authorities have said that sparks from the welding work, which was done without a permit, flew into the wooden clapboards behind 298 Beacon St. The building was engulfed in flames. Lieutenant Edward Walsh and Firefighter Michael Kennedy who were among the first responders to the fire, were trapped in the basement and died. Callahan’s estate has also named the owner of the building where the welding work was being done, Oliver Realty, Inc., in its lawsuit.