JWST observes the assembly of a massive galaxy at <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>z</a:mi> <a:mo>∼</a:mo> <a:mn>4</a:mn> </a:math>

We present JWST observations of the radio galaxy TGSSJ1530+1049, spectroscopically confirmed at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>z</mml:mi> <mml:mo>=</mml:mo> <mml:mn>4.0</mml:mn> </mml:mrow> </mml:math> . NIRCam images and NIRSpec/IFU spectroscopy ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>R</mml:mi> <mml:mo>=</mml:mo> <mml:mn>2700</mml:mn> </mml:mrow> </mml:math> ) show that TGSSJ1530+1049 is part of one of the densest-known structures of continuum and line-emitting objects found at these redshifts. NIRCam images show a number of distinct continuum objects and evidence of interactions traced by diffuse emission, and the NIRSpec IFU cube reveals further strong line emitting regions. We identify six continuum and four additional strong Halpha emitting sources with weaker or no underlying continuum within the 3’‘x3’’ IFU field. From spatial alignment with high-resolution radio data and emission line profiles, the radio AGN host galaxy is clearly identified. The bright Halpha emission (but not the optical components) is distributed remarkably linearly along the radio axis, suggestive of a biconical illumination zone by a central obscured AGN. The emission line kinematics indicate jet-gas interactions on scales of a few kpc. However, due to large relative velocities and presence of underlying continuum, the alignment with the radio structure appears to be, at least partly, caused by a particular configuration of interacting galaxies. At least four objects within a 10x10 (projected) kpc area which includes the radio source have high stellar masses (log( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⋆</mml:mo> </mml:msub> <mml:mi>/</mml:mi> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mo stretchy="false" form="postfix">)</mml:mo> <mml:mo>&gt;</mml:mo> <mml:mn>10.3</mml:mn> </mml:mrow> </mml:math> ) and star formation rates in the range 70-163 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:math> yr. Using a stellar mass-based analysis, we predict a total dark matter halo mass of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>≈</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>13</mml:mn> </mml:msup> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> . Based on the physical separations and velocity differences between the galaxies, it is expected that these galaxies will merge to form a massive galaxy within a few Gyr. The system qualitatively resembles the forming brightest cluster galaxies in cosmological simulations that form early through a rapid succession of mergers.