How to correctly merge the value in the columns, in the case of using the widget type the Value map, with QGIS

Trying to do work that was previously done in ArcGIS. There is a table when filling 4 columns, these values are added in the Sum column.

In QGIS, I decided to use the Value Map for fields

For the Sum field, I use the following expression

But as a result, I lose zeros
before number

Where did I go wrong and how do I get values with zeros when field type is real and SUM field is string?