Imagen 4 has no 4:5 ratio: get Instagram portrait from 3:4 on Sume

Imagen 4 Fast and Imagen 4 Ultra on Sume do not list a 4:5 aspect ratio, so you cannot request Instagram portrait directly. Their aspect-ratio options are 1:1, 16:9, 9:16, 4:3 and 3:4, and they take no custom pixel size. The workaround is to generate at 3:4 and crop 90 pixels from a 1080 x 1440 result to get 1080 x 1350, or to pick a model that does list 4:5.

Sume's docs describe 4:5 as Instagram portrait at 1080 x 1350 and warn that it is not 4:3. Models that accept it include Nano Banana 2 and Pro and, through a custom size, GPT Image 2.5 and GPT Image 2.

Which models list what

The catalog is the authority; the table summarizes the aspect lists the Sume docs state for the models most often used for portrait stills. Read each model's supported_parameters.aspect_ratio from GET /v1/images/models before pinning a ratio, because a value the model does not list returns 400 unsupported_parameter.

The 3:4 crop

3:4 is 0.75 and 4:5 is 0.80, so a 3:4 image is slightly taller than needed. Trim equal strips from the top and bottom: from 1080 x 1440 that is 45 pixels each side, leaving 1080 x 1350. Prompt for the crop, not just for the ratio: keep the subject and any text out of the outer 4 percent at the top and bottom, or the trim will clip them.

from PIL import Image

def crop_to_ratio(path, out, ratio_w, ratio_h):
    im = Image.open(path)
    target = ratio_w / ratio_h
    if im.width / im.height > target:       # too wide: trim the sides
        w = round(im.height * target)
        box = ((im.width - w) // 2, 0, (im.width - w) // 2 + w, im.height)
    else:                                    # too tall: trim top and bottom
        h = round(im.width / target)
        box = (0, (im.height - h) // 2, im.width, (im.height - h) // 2 + h)
    im.crop(box).save(out)
    return box

if __name__ == "__main__":
    Image.new("RGB", (1080, 1440), (40, 90, 160)).save("imagen_3x4.png")
    print(crop_to_ratio("imagen_3x4.png", "ig_4x5.png", 4, 5))
    print(Image.open("ig_4x5.png").size)

Cost and resolution

If you deliver both feed and story crops from one render, generate at 9:16 instead and crop the center to 4:5 for the feed: a 1080 x 1920 frame trims to 1080 x 1350 by removing 285 pixels top and bottom, which is more cropping, so keep the subject centered.

Imagen 4 Ultra also takes a resolution of 1K or 2K, which Fast does not list. A 2K 3:4 result gives you room to crop to 4:5 and still deliver 1080 x 1350 without upscaling. Both Imagen rows are text-to-image only, so they reject input_references; if you need to edit a product photo into portrait format, use an edit-capable model rather than the crop route.

Compare prices from each model's endpoint record, not from memory: the endpoint's pricing lines are the figure you will be billed against. The crop itself costs nothing, so a 3:4 Imagen render cropped to 4:5 costs the same as a native 3:4 render.

Two checks before a batch. First, confirm the crop preserves the subject by previewing three results rather than one, since a prompt that works for a single portrait can crowd the top edge in another. Second, keep the uncropped 3:4 original; if the client later wants a story or a 3:4 pin from the same render, you will want the extra height back.

A note on FLUX 3 Image

BFL lists 21:9 to 9:21 for FLUX 3 Image's video and describes wide image formats at up to 4K; its docs pages I read did not give an image ratio list, so I will not claim 4:5 for it. Sume does not list FLUX 3. For 4:5 on a model Sume serves today, the aspect-ratio changer post and the Image API docs are the places to start.