I'm a little surprised that it only takes 5 builders on the right side. (That said, I haven't looked at that level in ages, so maybe it's obvious when you get down to sketching out all the possible miner routes.) Does that include the two builders you need at the end to deal with the miner? Did you use more than one miner on the right side?
I used five builders and two miners to save the right two entrances, and I used used pretty much the same to get the left two entrances to join them.
Using the picture from The Lemmings Encyclopedia (I couldn't find a better one), I'm going to number the entrances 1-4 going from left to right.
First, a lemming from entrance 3 mined right at a certain place. He landed in the space below the space between entrances 3 & 4, but not before removing a lot of terrain near the ceiling. It then required only three builders to make a passage for the next miner. Next, a lemming from entrance 4 mined (also in a certain place). Using the bridges built earlier, he would continue mining all the way to the exit. He ended up in a position similar to where the last miner is in the picture above. As this miner was taking his last stroke, I had him build, preparing me to set up the direct drop trick.
As for the left side, I was left with seven builders, two blockers, and two miners. I ended up getting it to work with five builders and two miners, although not exactly easily. I had a lemming from entrance 1 mine near the right side of his space, and then I placed a miner in a certain spot so that he both freed entrance 2 and could get down to join the others with as little difficulty as possible. Unfortunately, this required me to start the miner on a build brick, and it takes several builders to reach the desired position.
The miner was just high enough to free entrance 2, but unfortunately was still a bit too high, and he stops mining near the bottom of the space below entrance 2. Fortunately, some lemmings can enter the space ahead of the miner due to the way the terrain is set up, and they can quickly prepare a route for him by filling the bottom-left corner of this space so that the miner can get through.
Despite the pixel-perfect positioning of several features, the solution is surprisingly not too difficult to pull off. The hardest part for me is laying down the bricks in the space below entrance 2 quickly enough before the miner got there.
Sorry I don't have a replay to show. Hopefully the picture and explanation will make it somewhat clearer.