4DPO would be too early for implantation unfortunately - essentially until at least 7dpo, the embie isn't developed enough for implantion. i got horrific (like doubled over) implantation pain for each pg i've had - it was very distinct - wasn't like a cramp, was more like a stabbing pain. for the single sided pain, one thing it might be is your ovary being a bit swollen and forming a lovely little corpus luteum cyst. you'll find each cycle you create a lovely little progesterone producing cyst that will, if pg, support the pg through until placenta takes over - the cyst can end up being large and painful, and cause the ovary to swell a little (or in my case, grow to 20+ times it's normal size, but mine was forced ovulation with drugs which helped - or hindered!)

you do become so much more aware of every twinge in your body when actively ttc... one thing to keep in mind (which my ob only explained to me this week) is that your uterus contracts (like braxton hicks) all the time - whether pg or not - and when you're concentrating on what is going on down there , you'll notice it more!

hope it's a good kinda niggle for both of you - good luck

BG