Volunteer group Friends of Horsey Seals said sightings were reported in the last fortnight in the Horsey area.
The latest case follows three previously reported incidents where a seal was spotted with a plastic disc around its neck.
The volunteer group helped rescue those three incidents, but only after the animals weakened enough to be caught.
The four animals seen in the recent sightings remain in the wild.
The charity’s research showed a rise in incidents from 473 in 2015, to 579 in 2018.
The report said certain animals are being affected by plastic, with a fourfold rise occurring with seals. 28 such incidents were recorded in 2018 compared with five in 2015.
In a bid to tackle the rise, local volunteers have launched a leaflet campaign to encourage people to take home flying rings that could harm seals.
The plastic objects restrict their growth and cuts into their flesh.
One seal, named Mrs Frisbee, was rescued in 2017 and released the following year.
A second, called Pinkafo, was rescued last December and released in May this year.
A third seal was named Sir David Attenborough, whose Blue Planet II series raised awareness of the environmental damage caused by plastic pollution.
The disc was cut off Sir David’s neck by a vet using surgical scissors before being released back into the wild in July after three months of supervised care.