how to convert if have polygon with four points?
for example:

{
"version": "4.5.6",
"flags": {},
"shapes": [
{
"label": "bro",
"points": [
[
676.686567164179,
708.9552238805969
],
[
687.1343283582089,
971.641791044776
],
[
666.2388059701491,
973.1343283582089
],
[
651.3134328358208,
710.4477611940298
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "r",
"points": [
[
585.641791044776,
194.02985074626864
],
[
628.9253731343283,
1020.8955223880596
],
[
609.5223880597015,
1019.4029850746268
],
[
570.7164179104477,
191.04477611940297
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "r",
"points": [
[
570.7164179104477,
189.55223880597015
],
[
609.5223880597015,
1022.3880597014925
],
[
591.6119402985074,
1020.8955223880596
],
[
554.2985074626865,
189.55223880597015
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "r",
"points": [
[
549.820895522388,
189.55223880597015
],
[
590.1194029850745,
1020.8955223880596
],
[
534.8955223880597,
1014.9253731343283
],
[
499.0746268656716,
201.49253731343282
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "r",
"points": [
[
499.0746268656716,
197.0149253731343
],
[
533.4029850746268,
1014.9253731343283
],
[
482.6567164179104,
1011.9402985074627
],
[
445.3432835820895,
213.43283582089552
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "r",
"points": [
[
445.3432835820895,
210.44776119402985
],
[
482.6567164179104,
1013.4328358208954
],
[
428.9253731343283,
1007.4626865671642
],
[
397.5820895522388,
219.40298507462686
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "r",
"points": [
[
396.08955223880594,
219.40298507462686
],
[
430.4179104477612,
1011.9402985074627
],
[
312.5074626865671,
1013.4328358208954
],
[
276.6865671641791,
225.3731343283582
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "r",
"points": [
[
273.70149253731336,
225.3731343283582
],
[
314.0,
1013.4328358208954
],
[
263.25373134328356,
1014.9253731343283
],
[
222.95522388059703,
222.38805970149252
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "r",
"points": [
[
221.46268656716416,
220.89552238805967
],
[
261.7611940298507,
1016.4179104477612
],
[
245.3432835820895,
1016.4179104477612
],
[
203.5522388059701,
220.89552238805967
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "d",
"points": [
[
591.6119402985074,
195.52238805970148
],
[
202.05970149253733,
223.88059701492537
],
[
208.0298507462686,
289.5522388059701
],
[
590.1194029850745,
249.2537313432836
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "p",
"points": [
[
590.1194029850745,
249.25373134328356
],
[
208.0298507462686,
292.5373134328358
],
[
218.47761194029852,
553.7313432835821
],
[
600.5671641791045,
528.3582089552239
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "i",
"points": [
[
606.5373134328357,
546.2686567164179
],
[
219.97014925373128,
574.6268656716418
],
[
227.43283582089543,
676.1194029850745
],
[
609.5223880597015,
647.7611940298507
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "d",
"points": [
[
609.5223880597015,
677.6119402985074
],
[
227.43283582089543,
705.9701492537313
],
[
231.91044776119395,
802.9850746268656
],
[
616.9850746268656,
774.6268656716418
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "credit",
"points": [
[
618.4776119402984,
789.5522388059701
],
[
230.4179104477612,
814.9253731343283
],
[
240.86567164179098,
937.3134328358209
],
[
622.9552238805969,
913.4328358208954
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
},
{
"label": "b",
"points": [
[
625.9402985074627,
925.3731343283581
],
[
243.85074626865674,
950.7462686567163
],
[
249.820895522388,
1043.2835820895523
],
[
630.4179104477612,
1020.8955223880596
]
],
"group_id": null,
"shape_type": "polygon",
"flags": {}
}
],
"imagePath": "3386991260N_document_three.jpeg",
"imageData": "B8PrZATTfNMw69loEf/2Q==",
"imageHeight": 1280,
"imageWidth": 720
}

I had an issue reading the JSON file. I think it isn't a good idea to use lists to read each lines in this case.
Therefore I changed the code and I suggest you to prefer dictionary research to retrieve the points (also it is less expensive because it is not necessary to read the picture).

In my case, I use rectangle bounding boxes and just one detection class :

with open(txt_path) as json_file:
        data = json.load(json_file)

        w = data["imageWidth"]
        h = data["imageHeight"]

        for idx, shapes in enumerate(data["shapes"]):
            x1 = shapes["points"][0][0]
            y1 = shapes["points"][0][1]
            x2 = shapes["points"][1][0]
            y2 = shapes["points"][1][1]

            cls = str(0) # just one class in my object detection project

            xmin = min(x1, x2)
            xmax = max(x1, x2)
            ymin = min(y1, y2)
            ymax = max(y1, y2)


            b = (xmin, xmax, ymin, ymax)
            bb = convert((w, h), b)

            txt_outfile.write(cls + " " + " ".join([str(a) for a in bb]) + '\n')