ank55,
Unless you tell it otherwise Project will automatically "peanut butter", (spread linearly), resource hours over a task's duration. That's why Dev 3 is overallocated on the first 4 days. In order to achieve your intent, you need to manually enter Dev 3 work
over the 5 days. One way to do that is via the Resource Usage view. In the Work field in the timescaled data, (right side of screen), enter 0h for Dev 3 on the first assignment (i.e. HTML Prototype task), for the first 4 days and 8 hours on the 5th day.
You didn't mention if your intent is to have a fixed amount of work on either task. Project will use the work equation to adjust duration, work and units depending on various factors. You might want to check Project's help file under the topic "work equation".
You may also want to take a look at MVP Mike Glen's tutorials which can be found at, http://project.mvps.org/links.htm.
Hope this helps.
John