This grows wild behind my house. It has many yellow flowers per "branch", and many branches. The flowers bloom after each other. The flowers have 4 petals. Each seed pod has a flower on top of it.
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As the previous answer suggests (vote given), its Evening Primrose, also known as Sundrop or Evening Star - Oenethera biennis to be precise - seeds itself freely. This native, naturally occurring one has 4 bi lobed petals, as in your picture, although these days, it's possible to buy 'new and improved' varieties created by breeders.
The pods left behind by spent flowers are tapered towards the end, three quarters to one and a half inches long, with 8 tiny erect lobes at the tip, black seeds inside if it's this plant.
Seed pods, green and dry, and ripe seeds:
Flowers, and entire plant:
That looks like an evening primrose in the second photograph. If those green pods form after the blossom is pollinated, that would be a clue.
Reminds me strongly of some species of Erysimum. This genus contains the common wallflower.
I think the flowers, buds, seedpods, growth habit, and leaves match, but your picture isn't the greatest. :)
I found a comparison picture that looks very similar: