I think something is getting lost in the translation here.
What do you mean "they just take up 4 minutes"? What takes up 4 minutes? Each picture has a duration of 4 minutes in the browser? All the pictures together take up 4 minutes of the timeline? You need to be much more clear and specific.
You need to delete all the pictures from the browser and start from scratch. If you carefully follow my instructions, when you list the imported files in the Browser window, each picture will have a duration of exactly one frame (per step 3). When you drag the folder to a 30 fps timeline, you should get a sequence exactly 16 seconds 20 frames long for 500 pictures.
Don't take any short cuts because you think you know the process. I verified the steps and they do work. If they are not working for you, then you have skipped a step.
Please repeat ALL the steps I detailed, exactly, and get back to us. If you are still having problems, please be VERY specific with your description of the problem. Do NOT use pronouns like "they", "them", "it", etc.
For example "When I drag a single picture into the sequence timeline, the picture's duration is exactly 1 frame long".
Not being specific will only cause confusion and frustration on both ends.
Ta