I use extension cords from my GFCI outlet, though I'm sure it's not officially "up to code". As long as the connection is kept dry it works ok. Wrapping it in silicone self-fusing tape (such as Rescue Tape) is a good way to keep it rainproof, or keep it in a plastic shoe box with little cut-outs for the cords, ensuring that rain and snow can't get in. Such a connection is NOT submersible, and neither are any modifications to the pump's cord (which would almost certainly void the warranty.)

A word of caution: If the pump is directly on the bottom of the pond and the discharge hose is running anywhere outside the pond (say, to a waterfall), you run the risk of emptying the pond if the water flow accidentally becomes diverted.

I use an extension cord meant for use outdoors. I have the connection point in a piece of pvc pipe, with the openings tightly blocked with foam and waterproof plastic. I think you are supposed to bury the extension cord, and enclose it entirely in some sort of pipe, but I haven't done so yet. I have it held securely out of the way so that it can't be pulled, moved, or tripped on.

If you have a lawn mower that you run anywhere near the cord/pond, you will want to bury it so you don't accidentally nick it.

My pump cord is 15' long. Are you saying yours is only 4'?

I use a large gauge extension cord. I think its something like 12 or 14 gauge. (The smaller the number, the bigger the wire). I've used this for years, attached to a GFCI, and haven't had any problems. As was mentioned, don't run over it with a mower. Oh, and if you don't cover up the connection, rain or snow might trip your GFCI, but that's no big deal, if you keep your eye on the pond daily.

Depends on how handy you are and how the pump is put together.

One solutions would be to buy an out door extension cord with molded plugs. Cut one end off and strip the wires. Take the pump apart and remove the old cord and install the new cord.

I would have to think about this for a little more, but you may be able to hard connect the extension and the old cord and then seal the splice in a plastic or something similar. I assume you could put the splice in a tube filled with a good wax. This is an un-thought through idea, so should be researched some more

You could always use marine grade heat shrink which you can get at Harbor Freight.