Ultracold polyatomic molecules offer intriguing new opportunities [1] in cold chemistry [2, 3], precision measurements [4], and quantum information processing [5, 6], thanks to their rich internal structure. However, their increased complexity compared to diatomic molecules presents a formidable challenge to employ conventional cooling techniques. Here, we demonstrate a new approach to create ultracold polyatomic molecules by electroassociation [7, 8] in a degenerate Fermi gas of microwave-dressed polar molecules through a field-linked resonance [9–11]. Starting from ground state NaK molecules, we create around 1.1×103 tetratomic (NaK)2 molecules, with a phase space density of 0.040(3) at a temperature of 134(3)nK, more than 3,000 times colder than previously realized tetratomic molecules [12]. We observe a maximum tetramer lifetime of 8(2)ms in free space without a notable change in the presence of an optical dipole trap, indicating these tetramers are collisionally stable. The measured binding energy and lifetime agree well with parameter-free calculations, which outlines pathways to further increase the lifetime of the tetramers. Moreover, we directly image the dissociated tetramers through microwave-field modulation to probe the anisotropy of their wave function in momentum space. Our result demonstrates a universal tool for assembling ultracold polyatomic molecules from smaller polar molecules, which is a crucial step towards Bose–Einstein condensation (BEC) of polyatomic molecules and towards a new crossover from a dipolar Bardeen–Cooper–Schrieffer (BCS) superfluid [13–15] to a BEC of tetramers. Additionally, the long- lived FL state provides an ideal starting point for deterministic optical transfer to deeply bound tetramer states [16–18].