Firstly, I must say that I very much like the fact that you can stand in place and get up to 16 (20?) seconds of swiftness (minus standing time.) That being said, much of the time Symbol of Swiftness can only be used by running through it and receiving only the initial pulse. This may be because you need to run for your life or it may be because you need to catch up to your target. By making the initial pulse 4 seconds only, the skill fails in this important area.
I would like to suggest front loading Symbol of Swiftness to it’s previous length of 8 seconds, then remove a second from each other pulse, meaning 17 (8 + 3 + 3 + 3) seconds of swiftness if you decide to stand for the full duration. In this way, it succeeds in being a get away ability while minimally penalizing players who already have swiftness before standing on the symbol.