How to get the assembly name of a custom web part in SharePoint 2010?

Here the way you can get the 4 part assembly name of your custom web part.

First of all add the web part to the SharePoint site by edit the page and click on the add a web part link on your web part zone. Once you will add the web part, go to Edit web part as shown in the figure below.

Then from the web part properties go to Advance and then in the Export mode Select Export all data as shown in the figure below.

Then click on OK.
After that click on Export... as shown in the figure below.

Then it will ask you to save the web part. Just save the webpart and after that open that file in notepad. It will give you everything about the webpart.

I got this information from OnlySharePoint2010