Add four images to this web page using the <img> tag. They should all be related to each other on some theme. The height should be 150px and width should be 100px.
Arrange the four pictures into two rows of two pictures.
Separate the rows using a <br> tag.
Make sure to put everything in the <body> section of the web page.
Sure, here's a sample code to add four images in two rows of two, with a height of 150px and width of 100px:
```
<!DOCTYPE html>
<html>
<head>
<title>Four images on a webpage</title>
</head>
<body>
<h1>My four related images</h1>
<img src="image1.jpg" alt="image 1" height="150" width="100">
<img src="image2.jpg" alt="image 2" height="150" width="100">
<br>
<img src="image3.jpg" alt="image 3" height="150" width="100">
<img src="image4.jpg" alt="image 4" height="150" width="100">
</body>
</html>
```
Replace `image1.jpg`, `image2.jpg`, `image3.jpg`, `image4.jpg` with the actual image filenames and extensions that you want to use. Make sure that the images are located within the same directory as your HTML file. Additionally, you can add captions or titles using the `alt` attribute.