Mandatory field on CRM Form

I have four fields on CRM form which are set as "Business Required". They  have been arranged sequentially on form. When I click Save, system puts the focus on Field 1, then on Field 2. But after that it moves the focus directly to Field 4. It skips field 3. Field 3 and Field 4 are Lookup and I have not even set any filtering on these lookup.

Any idea why system is behaving this way?

Thanks in advance.