On the other hand, worth of lend-lease is not just in its scale. Some things may have been more critical than others, depending on when they arrived, did the USSR have the means or the technology to produce them etc... Unfortunately, to really assess it, one would have to look at technology available in the USSR, their production capacity for various piece of equipment, rare materials production, their supporting industries (for example, the state of chemical industries) and I don't have the time, resources or desire to do that.
A good look at the goods seems to support mostly this view. Personally I don't think that the Lees and Stuarts were bad for the timeframe. We are talking here about 1941 and 1942, where the need for tanks was quite desperate in the Sovietunion. The Lee outclassed the MI, MII and was overall in the league of the MIII and MIV - that the preception was not overwhelming is due to the fact that the T-34 was just such though yard stick. The Stuart was still a lot better than the MI and MII and was good design, even if too light for late 1942 and 1943.

Overall I would say that the impact was a lot higher than the 4%, especially since quite some of it arrived when the Sovietunion needed it most and it where often critical goods and components. The relative impact might have been up to 15% - we wil never know.

What I don't really know is how close were the Soviets to the bottom of the barrell. Obviously, their population has never really recovered from the Stalin era, but I'm not sure that means they could not have fielded enough replacements to keep that large and well-equipped force they had in 1945 in the field.
Glantz wrote that in 1944 the Red Army was nearing the bottom of manpower. Given the immense military and civilian losses of the Sovietunion and of some conquered satellites this is all too understandable. A war of attrition between the "Soviet" alliance and the Western allies would most likely been won by the latter. But given the great relative advantage on the field IIRC at least 2-1 in manpower and also in material it is hard to know if to which extent the greater ressources of the West could have been brought to bear.