I'd recommend getting an engineer involved. Two years ago we had a builder knock a wall and fit an RSJ over a 4-metre span. Two years later, due to a fire in the house, we found out that the RSJ hadn't been seated correctly, and was borderline on being too small for the job.
This time around we weren't prepared to take chances. The engineer cost about €800 and €2200 for the builder to remove the old RSJ and fit a new bigger beam. The difference in the two jobs is substantial. And we now have a certified alteration, and the confidence that it hasn't comprimised the house.