The calling out of your name & the 4-digit serial number is for the POLLING AGENT (the witness from each political party) to mark against their name list so that there is a check and balance that each eligible voter votes only once.
The serial number behind your voting slip is only for "authenticity" purposes in case there is a dispute on the genuineness of the slip.
Think about it...
1.22 million voters, slips are counted in front of the potlictical parties, boxes are sealed & signed and kept in a vault for 6 months, no one allowed to open it unless there is a dispute and then infront of the political parties again. At the end of six months, the political parties inspect the seals and follow it to Tuas for burning. The only time the slips are seen by people is when is counted between 8pm 6 may to next day (at the most 24 hours).
...where got time to "scan" the slips? It takes about 340 hours (nearly 2 weeks) to scan them 24X7 assuming one second per slip. And where got chance to "open the box" without the opposition knowing.
YOUR VOTE IS SECRET
The myth of the voting slip has been around for the last 30 years... even before scanners