New user 
here, and straight to the point.
I have an HP nx9420 which right now I am servicing and doing a latest upgrade to it (4 GiB of RAM, out of which only 3.375GiB can be used due to 945PM 32-bit chipset limitations, and a Core 2 Duo T7400 cpu in the mail right now).
Still didn’t try Manjaro because I made a bootable USB memory with UNetbootin, which didn’t work (also tried Etcher, and it killed one of my USB sticks). I know the laptop boots fine via USB, as I have a Windows 10 USB bootable which works with it (I made it with 
Rufus) although it complains about my laptop not being UEFI and refuses to continue booting (guess I have to make an MBR/BIOS bootable, so).
I did read about Manjaro running nicely on old or low specs hardware. Hope it works for this machine which I still love so much.
I’ve barely used Linux, but I use an extremely tweaked Android build and perfectly know what directories, partitions, command line and related stuff is.
If someone is willing to pinpoint some hint before straight jumping into the pool with Manjaro please do so. Thanks.
I belive manjaro 32 bits dropped support a while a go, so I think that’s the reason it won’t boot.
I too have a so old laptop (Dell Inspiron of 2006) as a spare machine and it’s now working with Debian. Say what you will about grandma Debian, but she is very faithful to old acquaintance. 
4 GB RAM is the main problem of your system and a 32-Bit distro will fit your needs. A lightwieght GUI like XFCE is way better than plasma or gnome what i don’t recommend at this low specs. you’ll get a nice working oldie if you hang on these specs: 32-Bit distro and xfce. there are still some distros out in the linux universe that focus at especially old systems like this one. take your time, the link is to a popular site where you can get a lot of valuable information and you can use their search function to figure out what you are looking for.