Found the culprit, turns out to be a faulty receiver.
Have contacted Banggood about it.
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Make sure the receivers say V2 if they're FGr4 or FGr4S receivers.

Yepp, its a V2 FGr4S

Hi, I disassembled the NB4 and the receivers and connected a second antenna on each port. Based on this, I designed a external antenna connector.
(EDIT: would like to post the URL and some pictures, but not allowed yet. Workaround: Search on thingiverse.com for 4865056)

I tried several antennas; longer and shorter, circular polarized, directional antenna, also connected to a magnetic antenna holder with a longer cable which was placed on a higher position.
Unfortunately, I could not see any improvement whatsoever. So at the end, I'm using a 5 dBi which could be positioned more precised compared the the build-in one.
I also purchased a longer antenna for the receiver and connected it to the additional port. Again with no remarkable improvements. :-/

Receiver
 

hello, can i use ftr10, ftr8b or ftr12b receiver in rc car? what from one to the other is just the number of channels?

as long as you use a radio with -Protocol: AFHDS 3

Parts are now available to buy at: https://shop.flysky-cn.com/collections/parts/nb4

No need to go through support.

Q for guys who race, what is better fgr4s or fgr4p?

They perform the same. I prefer the FGr4S because it has a built in antenna, but I'm using the FGr4P in my Sworkz because the FGr4S wouldn't fit in the radio box. You might get some more range with the FGr4P and its external antenna.

Well it's not about range really, I tought more speed wise. I have fgr4s in my buggy and can fell a bit of a delay when gunning it of the line, but that can also be esc motor combo in that rtr. Just wanted to see what others say.

I'm using the nb4 for online racing with VRC. It's working fine, but has anyone tried it with Circuit Superstars? I did and I can't make the trigger correspond with throttle and brake. It will assign to other buttons like sw1-r but not the actual trigger like in VRC. Let me know if any of you had any success.

I assume it's one of the games that is set up for a Xbox controller (on PC) only. There are a few games where thats the only support they have. While I've not tried that game, that partial support reminds me of the issue I've had with other games. There's usually a work around, a google search says there is, but I didnt go further.

Binding issues, TX is on 2.0.99FCC version and I have a MXO-Racing RXC42-A3(NB4) RX. I can't get them to bind, what am I doing wrong?

have you updated it to .99? there's probably a set of pins that need to be jumpered to go into update mode, but I'm not sure about that model specifically. BeaverHobbies did a tutorial on the Flysky GMR, it may be similar to that one, but I'd be sure before jumpering pins

The BeaverHobbies GMR tutorial is probably the way to do it, my laptop is very old which could be the reason why it's not updating. I'll try a newer PC later this week and report back if it works.