Curved stems, afraid they fall, HELP!

How are you all? 1st post and first time to grow, and i need some advice. i got some haze X skunk seeds ( i know that its not the best choise of strain for the first time cause ive read and been told thats best for experienced growers ), bought a 400w HPS lamp, and i have a fan working with the light 18h day 6 h night. I've started giving them ferts, BNSoil Supermix and BNZYM .A week has passed and there are 4 plants and two of them look very healthy and are getting taller then the others, but the stem of these two doesn't look too thik, and its curving slowly, I've read that is good to have a fan working cause it makes the stem stronger, but do you think that the fan might be the problem? its not pointing towards them exactly its only a mild breeze.. could really use some help, take a look at the pics and hit me back, good smokin every one!

PS:Give me an opinion about the other two wich are growin a lot slower and i dont know if thats normal..

Link below:

http://i286.photobucket.com/albums/ll112/moobboom/DSC03316.jpg
http://i286.photobucket.com/albums/ll112/moobboom/DSC03327.jpg
http://i286.photobucket.com/albums/ll112/moobboom/DSC03324.jpg
http://i286.photobucket.com/albums/ll112/moobboom/DSC03313.jpg
http://i286.photobucket.com/albums/ll112/moobboom/DSC03309.jpg