Over Fert ! IMO Straight up!
I rarely hit 4ml of biobizz bloom a litre and you are supposed to be using 1 ml of bio grow right the way through the cycle, it contains allkindsa sugars and b1 to help your plant thrive.
Got a PH meter or EC meter ? if you dont go and get one or at least some PH drops. I will bet the PH of your runoff is below 6.2 possibly even as low as 5.8 and it will have an ec of anywhere around 2.4 ( I have grown a lot of biobizz and my first mistake was just like yours).
I could be wrong. At 3-4 weeks the typical deficiency is MG. I would say you have a lockout caused by too much P and K and it has dropped your PH massively. Did you add Lime to the allmix ? I highly recommend lime and perlite when working with allmix.
I use a 50/50 mix of bio bizz all mix and gold label special mix with an extra 20 percent perlite added. It rocks because the allmix is restricted on a lot of minerals that they can use to stay organic where as the Gold label special mix has swedish clay (mineral rich) and added lime it makes for a much more complete soil.
Do not take anything at face value and always ensure that the diagnosis is correct before you treat.
This is just the stoned ramblings of a guy who likes to grow pot. Take it or leave it
Mouse
EDIT : The smallest pot I run is a 7.5 and I have since abandoned them in favour of 10's and now I am abandoning 10's in favour of 15-20 litre air pots. You may want to find a bigger one. Check greengrocers and florists for buckets and drill holes in them, mine cost me 20 pence each. Costs me a quid for the bases lol