I thought my fourth finger was the weakest one but I discovered that my index finger was my weakest link. This is because 90 percent of the time guitar players keep the first finger down while the other fingers are always moving.
I watched this Shawn Lane video and he said to make a deliberate motion with the index finger when using it. The reason being is that the extensor muscles do not get developed properly with this finger due to lake of motion. This is especially true for hammer on technique. Try to do a hammer on with your index finger, it ain't as easy as the others..why is this??? Greg Howe practices hammers with his index and so did Hendrix, this is how he was able to play without a pick, just hammers...the index finger needs love too..
I realized my index finger was the weakest because when ever I tried to put any speed into my laying, the index finger would always trip me up. So now, as a warm up, I practice deliberate up and down motion with this finger, on an exaggerated level and since I have been doing this for a few months, my ability to speed up has improved.
Many people are skeptical about this idea but trust me, it works. If anyone wants me to show an example of what I mean let me know and I will do a short video.