Hey, Look how much sun are they getting and how much water, also...where are you in the world...cuz if aussie its winterish months so thats partially a problem...if your weed is going yellow at the tips of the bottom thats usually nitrogen deficiency, but it can be overwatering causing root rot and stuff, them two bottom leaves are nothing anyway but if it starts spreading ud wanna fix the problem, the way to do this is get some good fert jus a cheap one will do it, but check on the back at the ammount of nutrients, if its to strong ull kill the plant, look through the site or just email me and ill tell ya what they should have...
remember...
never go overboard, it is a weed and if you try keeping it good and doing to much it gets lazy and dies...neglect = best. every 4 days water it and onkly apply ferts 1nce a week...u should only place the fertilizer on the top of the ground soil around the plant then water it and put ur miricle grow on...theyre 4inches, 4weeks...sounds normal for an outdoor thats lacking...ull soon notice new sprouts everyday if you do it the way i have stated.
keep it real and take it easy
Chill out
peace.
PaPPa sMuRf - [email protected]