If I don't transplant the plant's what will probbably do? Can I fix the problem without doing this?
If the plants need to be transplanted and you avoid it, they will slowly become flimsy, they will slowly stop growing, and they will slowly die, wasting a lot of time....
If you do transplant them, have a big bucket of good water to put them in and untangle the roots.
11 liters doesn't seem too too bad for 4 small plants, to measure the roots, I like to look at my plant and picture it being upside down, the plant being the root ball in this prospective obviously.
If it doesn't seem it would be touching the bottom keep it in the same pot.
If you don't think it need transplanting, from here I would start feeding it HALF of what you normally mix, turn it to 12/12, trim of some shitty leafs if you see a lot of brown on them.
Make sure if you're using tap water to let it sit for 24 hours in order to lower the pH level.
Try reading some of the stickies we have in this section, they are very helpful.