As I understand it, the point of using root-trainers is to allow a really good root system to develop. Do you stand the root trainer container on e.g. a couple of flowerpots, to allow air to get all round? This is supposed to stop the tap root from coming out the bottom. Instead the roots start to grow upwards inside, and develop this really good root ball. It's that that allows the plant to be replanted with no set back at all. I've certainly found that to be so during the 8 or so years I've been using root-trainers.
I don't plant out until I've checked what the roots look like. You can lift out one line of four, lie it on its side, carefully open it and have a look. if the roots have more or less filled the earth, it is ready. If not, then the earth will fall off when you try to plant it, and it is too early!
Your sprouts sound a little on the small side, at 4 ins, for that to have happened yet. Be patient! I rarely plant out my brassicas until end May/June.