Which wifi router is suitable for 4 room hdb house?

Honestly, most routers that have a decent review online are good enough. 4 rm hdb are not that big.

The main issue is the placement. As a rule, the router should be placed in the center of the house, with line of sight into as many of the rooms as possible. The less the signal has to go through walls, the stronger the signal. Its also a good practice to place the router at around waist height and away from other heavy electronics (microwave, fridges, or anything that takes alot od wattage) as they can interfere with the signal over long distances.

If your access point is at the corner of your house and don't have ethernet cable points there, you can go for are meshes. The tp-link deco, Google or netgear ones have had consistently good reviews. I'm have been using the deco M5 for over 2 years, no issues with it.

I live in a 4-rm flat. To cover the nooks and crannies I highly recommend getting a mesh WiFi system instead of just a single router. I previously used an Asus Router and got patchy connection in the corners, but ever since I started using the TP-Link Deco M9 mesh system on a MyRepublic subscription in 2019, both the coverage and connection have been excellent. Absolutely no issues at all connecting whether I am in the service yard or toilet or corner of a bedroom.​​​