How to update a column with values from the same newly inserted row in a trigger? Using SQL Server 2014, I have this:

Table1
Tab1_id        int identity(1,1) primary key
Tab1_val1      smallint
Tab1_val2      smallint
Tab1_valInfo   varchar(10) -- not yet implemented, will be used only for presentation




 Tab1_id |   Tab1_val1 |   Tab1_val2 | Tab1_valInfo
---------|-------------|-------------|--------------
       1 |          25 |          19 | 0025-0019
       2 |           0 |           5 | 0000-0005
       3 |          12 |           3 | 0012-0003


Now with my pretty short SQL experience, what I'm trying is:  

From an application where the only input values required for inserting data are Tab1_val1 and Tab1_val2, then, create a trigger which updates Tab1_valInfo column.

create trigger utr_ValInfo on Table1
after insert as
begin

    declare @size int = 4;

    declare @valInfo varchar(10) =
        select right('0000' + convert(varchar, /* insertedRow.Tab1_val1 */), @size) from inserted
        + '-' +
        select right('0000' + convert(varchar, /* insertedRow.Tab1_val2 */), @size) from inserted;

    update Table1 set Tab1_valInfo = @valInfo where /* this is the last inserted row*/;

end;


A single insert may involve multiple rows.

I hope what I'm trying to achieve is clear.
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