The U. of Maine page gives a maximum length of 3.7 mm for Bembidion quadrimaculatum but this CBIF page gives a length of 5 mm for Lebia ornata. Compare images of both species here.
Some differences I noticed between the two:
In L. ornata the abdomen has a squarish shape and its widest part is at least twice as wide as the thorax, the posterior spots on the elytra are closer to the midline than the lateral edges, and are less than half the size of the anterior spots, and the tip of the abdomen protrudes beyond the elytra.
In B. quadrimaculatum the abdomen is more oval-shaped and its widest part is less than twice as wide as the thorax, the posterior spots on the elytra are closer to the lateral edges than the midline, and are more than half the size of the anterior spots, and the tip of the abdomen doesn't protrude beyond the elytra.
I have a poor-quality shot taken in New Brunswick that matches the description for L. ornata and looks virtually identical to Tom's photos. I estimate its length at no less than 5 mm, which is larger than the largest B. quadrimaculatum.

The black coloration and the markings look good for Bembidion quadrimaculatum, but I'm seeing the abdomen sticking out beyond the elytra. These IDs can be confusing with all the subtleties and variations. I'm confused.

So many things are running around with two names, and two groups of defenders no doubt.

Thanks for the confirmation Robin.

Duh! Wrote this not having read Robin's initial post. Mild redundancy ahead...:-) I agree with the Lebia diagnosis on this specimen. Note the truncated elytra (squared-off wing covers), one characteristic of Lebia, but not Bembidion. I also think the shape of the pronotum does not agree with Bembidion. Lastly, habitat is key. Lebia are much more likely to be encountered on foliage well above ground, often in the forest understory. Bembidion are abundant on the ground, especially near rivers and streams where the shore is muddy or at least very damp.