Upload & Sell: On
IMO, no other lens, however expensive or good it might be, is a sufficient reason to dump 400 f/5.6 which has a unique combination of qualities: very good IQ, very fast AF drive, low mass and relatively cheap.
I got the 100-400L which really only trades the super fast AF speed of the 400 for far more practicality and same IQ. While the 400L's weight was a delight, I can handhold the 300 + 1.4 for long stretches even at eye level and the extra stop speed, 1m shorter mfd, and IS were all worth the trade-off in weight. IQ is slightly better and AF speed is still excellent.