If you're looking to keep any kind of muscle, I would not recommend a T4 only cycle. Hell! I wouldn't recommend a T3/T4 ONLY cycle! Yes, fat will be burned, but muscle that has taken years to put on, can and will get burned right along with it, specially of you diet is not on point. I would use at least 500mg of a long ester testosterone such as Cyp or Enanthate and for safety 50mg of Anavar if you're dieting big time for further muscle retention, eat lots of protein, depending on your bodyweight you should know what that amount is, again I don't know what your goals are, bodyweight, fat %, how long you've been lifting seriously and how many cycles under your belt, but I can't think of 1 single reason other than you are seriously overweight to attempt such a cycle, even then is too risky to shift you metabolism up like that... My 2 cents...