I think 4 days would be too early for an implantation bleed... do you know when you ovulated? It could be an implantation bleed if you'd ov'd earlier than you thought.
Normally implantation doesn't really start until 7 days post ovulation (DPO) but can be as early as 5 days. Implantation is complete by 13 DPO so it's possible to get a tiny bit of bleeding any time around then.
If you only had the implant out last month, it is very early days and your hormones may not be back to normal yet. You may be one of the lucky ones though...
Not really, 4dpo is too early for implantation. 5 days at the very earliest, 8 days is apparently the most common. Of course you may have ovulated earlier. could be random spotting after coming off contraception which is quite common I think, and does not mean that you can't be pregnant. I'm afraid you'll just have to wait 10 days and see, fingers crossed!