Here's the history behind the Four Sword.
It began as the Picori Blade and was created by the titular species. It resembled the Master Sword, but was not the same blade. In TMC, it is reforged as the White Sword, and after acquiring the four elements and infusing them into the sword, it finally becomes the Four Sword.
No Master Sword involvement. Remember also, the Four Sword is featured in A Link to the Past also (in its Temple in the GBA remake of the game), so if ALttP takes place in the Child Timeline, then that would mean TMC has to take place before OoT in order for the Four Sword to appear in both timelines. However, if it [and it probably does] take place in the Adult Timeline, then TMC could very well take place at any time prior to FS/FSA, with the most logical placement still being before OoT.
So in conclusion: Four Sword ≠ Master Sword.