<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by number_6:
Luckily BA has such a lounge at each of those locations. And your choice is easy, there is one to pick from.</font>
At LHR T4 (which is where you will connect in both directions) there are actually two J class lounges. One is at gate 1, and is the bigger of the two. The other is opposite gate 10, and is the top floor of the lounge pavilion. The Molton Brown spa (to which you will have access on a BA J ticket) is on the bottom floor of the pavilion, next door to the First lounge.