I used 4mm to 5mm adapters that came with my Turnigy Graphene pack a little over a year ago and they worked great until one day I had intermittent issues with my ESC randomly shutting off. I had learned that my negative lead had become de-soldered over time so I re-soldered it and was good for about 5 min before the negative wire de-soldered itself again... which I re-soldered a second time and once again it became de-soldered. At that point I swapped out a fresh set of 5mm bullets and the pack ran fine the rest of the day. After closer inspection I could see that the 4mm to 5mm adapter had signs of oxidized corrosion on both sides of the adapter and it needed to be replaced. Unfortunately for me, the excess heat from the added resistance from the worn connector caused premature swelling of my pack and I decided to discard it.

If I were to use 4mm to 5mm adapters again in the future, I would probably get into the habit of replacing them every 6 months or so.