I just had a quick look. The Two-Way harnesses forward-facing to 25kgs, there are a couple of others on carseat dot se that FF with harness but only to 18kgs.
At 4 my DD1 is still only 16kg, so a harness til 25 kg will do her until she is at least 6. A 18kg limit would put her about 5.5 before she would go to a seat belt. She is super tall and went into a booster by 3 tjough as she was too tall for all the Australian seats.