A picture or drawing would be nice.....but if you have to prep 4 mm at 45°, then it becomes a groove weld...
If I'm picturing this correctly, you have a 5mm plate that is beveled 4mm at 45° leaving a 1mm land. I'm assuming the weld will be placed in this 4mm deep, 45° preparation, so if there is no melt-thru, it is a partial penetration joint(PJP), if there is consistent melt-thru with re-inforcement on the back side, then it is a complete penetration joint(CJP).