Do as OnlineDM recommends. Start them with adventure 1, but cut out some of the combat encounters. There is a lot of extraneous combat that adds little to the story and is just there to level the characters up faster. You can easily cut much of it, especially in the first two modules. The level gap will disappear shortly. Encounters in the first few modules need to be adjusted for MM3 math updates in any case, so it should be easy to tweak things for their level.
When I did my own conversion to 4E from the 3.5 version, my group came out of module 2 at a much lower level than the official conversion. They added a lot of stuff solely for the purpose of making the group level faster.