Which is better: DIY invest @ S$100 /month in each of 4 ETF RSP by Nikko-Am (STI , ABF, REIT, Corp. Bond) for a year on DBS invest saver OR digiportfolio Asia for $4000 at once, if I can afford both?

Have you considered other robo-advisors? Many offer far more diversified portfolios than the 2 options you've listed here

It is best to invest in a lump as it will give higher returns then dca most of the time. Also, it will be good for you to invest in global equity as it can give higher returns.

I will say lump some investing 8/10 times because money invested will almost always bring you better results.

If you are worried about entry price , maybe you can DCA every quarter instead of every month

Compared to digiPortfolio which only has 2 portfolios to invest in, I rather choose the OCBC Roboinvest which gives you 20-30 portfolios to choose from. I've invested for 2 weeks and I'm up 3%. I'm monitoring the situation to see if it's because of the market rally or FX before putting in more money.

DBS's digiPortfolio only has 2 portfolios, one local one US. The latter requires you to have a multi-currency account which is quite troublesome. For OCBC Roboinvest they cover US, HK, AUS and others.