Final answer:
The exact proportion of complex sentences used by TD 4-5 year olds is not defined, but their sentence length and complexity increase with age, as they progress from simple to more complex sentence structures.
Step-by-step explanation:
The proportion of complex sentences used by TD 4-5 year old children is not specified in detailed percentages or ratios. However, during this developmental stage, five-year-old children can typically produce sentences of five to seven words, and can define words by their function. They also begin to enjoy jokes and riddles, indicating an understanding of language nuances. As they approach six years of age, they start using complete but simple sentences, and by the age of ten, their sentence structures become longer and more complex comparable to adults. This suggests that while complex sentence usage is developing, it is part of a gradual progression that continues to improve with age.