在四年级下册英语第四单元的Let's talk部分，学生们通常会讨论一些与日常生活相关的话题。以下是一些可能的句子和对话内容：

问候和介绍

A: Hello! How are you today？

B: I'm fine, thank you. And you？

A: I'm good, thanks. What's your favorite vegetable？

描述物品

A: Look at these! Are they tomatoes？

B: Yes, they are. They are red.

A: What about those？ They look like carrots.

B: Yes, they are. They are orange.

询问和回答

A: What is this？

B: It's a potato.

A: How many potatoes do you have？

B: I have five potatoes.

品尝和评价

A: Try some of these green beans. They are good.

B: Yum! They taste great.

A: Do you like carrots？

B: Yes, I do. They are crunchy and sweet.

总结和提问

A: Today, we talked about different vegetables. What was your favorite？

B: My favorite was the tomatoes. They were very juicy.

A: We should eat more vegetables. They are healthy for us.

这些句子和对话内容可以帮助学生在Let's talk环节中进行简单的英语交流，练习日常对话和描述物品的能力。