Shaun Miller scored twice on his full league debut for the Blades with Neill Collins and Chris Porter also finding the net. Boro skipper Mark Roberts struck at the end of the first half to make it 1-1 at the break.
"It's an unpleasant feeling and we've sampled it too many times now already," said Smith "I thought when we got ourselves back in the game at 1-1 there would be a much greater desire to compete and to make it a bit of a contest.
"We failed pretty much all afternoon to deal with any delivery into the box. It's a feature of this Sheffield United team and they do it very well. The movement and bodies they get into the box offers a very positive edge to their attack and we were unable to deal with it.
"I have to say the back four have not stood up strong enough and we've paid the price for it.
"Our challenge at this point, while we're not playing well, is to be a side that's more difficult to beat. We've done it previously when we've shown good resilience, good shape and good discipline away from home."