Without a doubt, the back 4 are a big problem. Walker & Rose are a liability, neither can defend properly, they get caught out of position all the time, leaving the CB's isolated. They both give the ball away, especially Walker, and when they do bomb forward, their final ball is invariably poor. Dier is young and still learning. Verthonghen for me seems disinterested, and also lacks pace anyway, and is weak in the air. Fazio is good in the air, but slow on the ground.
This is a problem that's been going on for years, and if we are to push for top 4 next year, it needs sorting. Pochettino must also be able to see the weaknesses so lets hope Levy backs him in the summer, plus the midfield enforcer and striker or two that we desperately need.