Hi Zi007AD and welcome!
There are two techniques for inserting values into a table.
The first uses default values. Go to Data->Default Values. Add a bunch of group4s - say 10. Add conditional formatting to field3 to hide it if it's blank. Next, add a rule to the dropdown that sets field3. You will need to know which one to set. You can count the ones that are not blank using an xpath function. Or you could add a counter to your main data source and increment it in the rule after copying the value. To copy the value, you will need to set field3 but only the field3 that is next in line. This is called filtered set. To do this, you will need to export the source files (File->Publish) and manually modify the manifest.xsf to add the square bracket filtering on the target field of the field set action (something like field3[/my:myFields/my:counter] where the counter is a new field you add to the main data source, but it could be in a secondary data source). Save the XSF and right click and choose Design. Verify in the rule action that it is setting the filtered value. Save the XSN as a new file (file->Save) will save back to the exported source. Anyway, it's a little tricky to do but easy once you get the hang of it.
The othe way to do it is to use qRules insert command. This is a better approach because it doesn't rely on default data and lets you add a rule to field3 that will copy the value into it when it is inserted. So, it's easier to do. qRules is a collection of over 130 commands that help form designers extend InfoPath. It's a common library of code that wll work in any form and it is developer license based so it's cheap. Free trials are available at Qdabra dot com and there is documentation in the User Guide on how to use Insert.
Here's a related sample I did that shows both techniques when copying repeating data: http://www.infopathdev.com/blogs/patrick/archive/2011/09.aspx. In your case, you just copy one item when the dropdown changes. Another approach might be to use a checkbox. Ideally, you also want to filter the dropdown after you have selected one of the items so it doesn't show again. Easy to do with some xpath filters.
Good luck!