If the effect size of 1.33 for assessment-capable learners, indicates nearly four years of growth in one year, would that mean that a 1st grader would be essentially learning at the grade level of a 5th grader by the end of first grade?

This is the problem with any measure that uses ages. The age-old grade problem of saying a five-year-old has a reading age +5 implies that this five-year-old is actually an 11-year-old in reading level, which is not necessarily the case (but could be). I use the notion of speeding up the rate of learning by a factor of 2 or 3 – to help reduce this impressing that a five year old is working at the same level as an 11 year old – which is often impossible as the five year old has not been EXPOSED to the powerful content that is needed between five and eleven to make this the case. So, yes, the person is right to be cautious, but there are factors that speed up the rate of learning and this is our focus” (John Hattie, personal communication, October 20, 2018).