Are the government benefits difference for 4 rooms and 5 rooms HDB flats significant enough such that it is better to opt for a 4 rooms HDB instead in the long run?

Are your earnings potentially more than the grants? Is your need for space and privacy negotiable? Everyone is different. only you can answer it yourself

4rm receives more grant and assistance and demand for 4rm is also higher.

It depends on your needs and budget. For me, I would go for 4-room as it suits my needs. I will go for 5-room if I need more space. In terms of value appreciation, it depends on how many year lease the house Has, so just go for newer house with higher growth potential (check if there is any plan for near future in this area).

I assume you are referring all the government subsidies on Usave etc. You may get as a whole just $80 Usave rebates difference each year. However bear in mind, usually bigger the house, the greater the usage and utility bill. Consideration for 4 room is usually the size. 4 room has higher resale demand. If you have plans for bigger family then 5 room for more space. Lastly, it may sometimes be a budget constraint. 4 room is definitely more efficient.