How to update a column with values from the same newly inserted row in a trigger?
Try this

create trigger utr_ValInfo on Table1
after insert as
begin

    declare @size int = 4;

    UPDATE Table1 

    SET 
         Tab1_valInfo  = right('0000' + convert(varchar, i.Tab1_val1), @size) 
            + '-' +
             right('0000' + convert(varchar, i.Tab1_val2), @size)

    FROM Inserted i

    WHERE 
         Table1.Tab1_id= i.Tab1_id;
end;


and don't declare varchar in convert without length