I have a hard time picturing a boss on top of 1/8" plate with four "1/4-in. butt welds." Is the boss beveled 1/4" deep so that they are single-bevel groove welds? Or are these really fillet welds with 1/4" leg lengths? Can you describe it a little more? Also, what is the weld material type and welding process? As far as determining the load it will handle, a very good reference is Design of Weldments by Omer Blodgett. Go to the Lincoln Electric website to purchase a copy for $10.00 at:

https://ssl.lincolnelectric.com/lincoln/store.asp?PID=16&cat=7

This is an excellent reference at an extremely low price. Best $10 you'll ever spend. The load it will carry depends on what direction the load is applied in - tensile, bending or shear. The book will help you understand what those terms mean if you aren't familiar with them.

Hope this helps,
Marty