Tomatoes are usually smart enough to know when it is time to bear fruit. That's about maturity, and not necessarily size. If the plant can support fruit, the flowers will fertilize. If not, then they'll drop off. But realize that once it starts fruiting, it isn't going to grow a lot. Four inches is kinda small for a 6-week old plant, unless the temps have been low.

carol23 - Thank you so much. I think that was the fastest reply I've ever received on these forums. You're right that my plant looks just like the Solanum nigrum. And it seems the ripe berries are edible. Reading online, it seems this plant is commonly mistaken for the deadly nightshade plant.

This species self seeds all over my allotment. You don't harvest it leaf by leaf. Just cut off a whole plant. It needs thorough rinsing because being a winter salad it gets a lot of mud splash and can be gritty if not carefully cleaned. The leaves are larger on the basal rosettes. Once they start to flower the leaves get smaller. But they're perfectly edible at any stage, including the flowers. And superior to lots of other wild greens imo. There are commercially available cultivars with larger leaves.

It isn't a Physalis. It's a Euphorbia, as said above. There are pictures of similar plants online, but those are labeled E. graminea. This plant may not be covered by any of the US keys. I was hoping that someone could provide the species ID. This search for Euphorbia graminea photos shows some plants that are very much like the plant in the photos. Other pictures show plants with much more narrow leaves, which would be expected for something named "graminea".