Something to be aware of when enlarging any relatively low resolution original to something so large is that you are going to run into several issues that may, or may not, lead you to reconsider.
The first thing to be aware of is that regardless of how many DPI (dots per inch) you may be able to scan at, the original print will have a vastly lower effective screen resolution of typically no more than several hundred lines per inch effective, so the commercially printed original is not usually a good high resolution source and not usually a good enlargement source. What this means is that when you scan it at a high resoultion and then try to enlarge it you will typically just end up reproducing the actual printer screen details (that is, the halftone Cyan, Magenta, Yellow, and Black dots) of perhaps the equivalent to a few hundred effective DPI and the individual colored dots are what you will effectively be enlarging rather than the photo details themselves when you use a high scan resolution. This often results in the screen (dots, dithering, halftoning) characteristics becoming a dominant detail for anyone looking at the image from a reasonably close distance and may only look acceptable from afar and may prove objectionable depending upon expectations.
If, by chance, you actually have a photographically reproduced original (which is rather rare these days since most are digital reprints or even digital originals from a DSLR or digital back medium format camera) then you will not have the undesired printer screen effects, but the effective usable resolution is still seldom more than a few hundred DPI when scanning due to the fact that the enlarged print has already been through an optical enlargement step (from the original 35mm or medium format film original). This means that even though you may be capable of scanning it at up to 2400dpi, the source likely does not really have much more than 300-600dpi of useful resolution left in it (for example, even good quality relatively low ISO 35mm or medium format film seldom has more than about 2,000 dpi of resolution initially and so a first generation 35mm film original is being pushed hard to get much more than a 20” enlargement out of it without details becoming blurred or film grain becoming obvious, with larger films scaling proportionally). For this reason, large photographic blowups, whether digital or film, are often assembled from a tile of photos that are then stitched together to create a very high resolution original so that detail does not deteriorate from the enlargement step — you see this done a lot for wide panoramic shots.
In some cases you can get very good results if you are working with a relatively low resolution source by combining it with a fractal enlargement algorithm/utility. The fractal algorithm will introduce false, but usually very convincing, texture and detail and allows one to sharpen the edges so that a good looking large high resolution print may be made.
So before you spend real money having the postcard enlarged and reproduced, I would encourage you to first make a test print made from tiled letter sized pages, printing perhaps 1/4 of the source photo (a corner, for example) so that you can first determine if the result is something you will be happy with from the distance(s) you will typically be viewing it at before you spend the money on a professional photo print. Your dimensions are a bit vague, but estimating from your description you would be enlarging by around 300-400% to fit either the 4 foot or 1-1/2 foot dimensions assuming a 4x6..that is 4x18... postcard as the basis), which means that the effective reproduced photo resolution may be as low as 60-80lpi/dpi effective (note, effective RGB/CMYK print resolution is much, much, lower than a printers physical print resolution as it takes a rather large grid of CMYK dots to reproduce a given color from the possible 16.7 million color palette). You are more, or less, at the edge of what one may consider an acceptable result depending upon what one’s expectations are.
As always, good luck in your endeavour — and if enlarging does not work out you can still make a very nice, albeit much smaller, wall hanging from the original panoramic postcard as-is.
