You really don't need 360 PPI, PPI is the screen/file resolution the printer will be using DPI which is totally different.
There is a formula I use regularly for poster and canvas prints which is 1Mb of file size for every Inch of canvas on the longest side, this will give a file that will match what the printer needs. Any more than this will just be dumped by the printer diver. Any more than this will give a poorer image as PS will have to 'extrapolate' more to fill the file. I do it this way which guarantees the minimum of extrapolation.
In PS go to New, in the dialogue box make your units Inches and enter your 60 X 48, in the Resolution box enter the value that gives just over a 60Mb file which is 86. Press OK. You now have a canvas the print size you want at the resolution the printer needs.
With your new canvas on the screen at the resolution and size you need go into Bridge and Highlight the image you want, go to File>Place and PS will plop the image onto the canvas plumb centre, it will look small it will already have the Transform Tool selected. Holding down the Shift Key (to maintain Aspect Ratio) drag the corner 'anchor points' to fit your canvas, you can even use this to crop at this stage, you can move the image around by left clicking and dragging. It won't look good, hit the Enter Key and PS will do the 'interpolation', it will look a lot better.
You can inspect what will be laid down by the printer by double clicking the Zoom Tool (the magnifying glass icon) this will give you a 100% view, you can also use the 'Print Size' view which will be slightly smaller.
Now is the time to apply any sharpening, save the file, this is the file you take to your print house.
If you have your resolution at 360ppi it will make a massive 1.04Gb file, the point being that the printer driver will dump all but 60Mb of this (assuming the printer is using 300DPI or thereabouts), but PS will have to add loads of pixels to 'pad out' the file so there will be loads more 'invented' pixels.
I routinely print this size until recently from a 10Mp camera, the amount of detail is very good, a head in the distance just a few mm in size on the print will have shading and be recognisable. There are limits, but 5ft X 4ft isn't quite it.
I have printed billboard size pictures from my Pentax 645D with it's 40Mp sensor using this same technique.
The caveat to all this is that you image needs to be technically perfect, blown out to white or blocked out to black pixels which you would not notice on an A4 print will be amplified by the size and stick out like a sore thumb. It also tests the resolving power of your lenses, at this size any minor fault is amplified.
Chris