I am in Fiji with a Coleman 110v / 240v gen set and want to adapt my 4 pin female receptacle to accommodate Fiji's 3 pin 240v plugs. I know my get set is 60 hertz and the power tools here are 50 hertz.

Unfortunately there isn't a good way to convert from 60hz to 50hz. Voltage differences are easy to deal with, but unless a device is designed to operate on 60hz, it will require a frequency converter to operate on 60hz. A 60-->50 frequency converter is a 60hz motor feeding a 50hz generator. The conversion is inefficient and not suitable for small loads.

Many electronics today have power supplies that can work at multiple frequencies, but I doubt that your power tools will be compatible with 60hz. Your cheapest option is going to be either buying 120v/240v 60hz tools or buying a 240v 50hz generator, depending on what tools you need and their costs.

On 50hz devices that are safe to use at 60hz, you will find that motors operate 17% faster,and use 17% more wattage. These changes in performance can lead to other problems if not designed to handle the increased cycles. I know 60hz devices operated at 50hz have less power, tend to overheat, and generally have a greatly reduced lifespan.

As explained above, doing so may not work very well. If you want to try it, you need a 4 pin plug, some 3 conductor cable and a 3 pin receptacle. You might start with a short extension cord and cut the one end off and replace it with the end you need. Connect the ground to the ground, and the hots to the hots. You won't use the neutral just like 240 volt things here such as water heaters, A/C compressors, and pumps.

The other thing you have to watch for is how power is distributed in Fiji. Some countries distribute true 240 volt power - so your 3 pin connector would be hot, neutral, and ground. The US distributes two 120 volt lines that are opposite directions (when one is at +120v, the other is at -120v, so a hot is used in place of neutral, resulting in the device seeing 240v). If Fiji distributes true 240v power, you may need a voltage converter.

Using the second hot in place of neutral for a purely resistive device (like a heater) would make no difference, but I'm not sure if all devices are fine having neutral replaced by a hot wire (and getting half the voltage off each hot).

Except for the frequency difference, I think the wave form of the 240 off a grounded, center tapped winding would be the same as one without the center tap and one of the 240 poles grounded. The only difference would be if one pole was grounded to a metal housing. I don't know about Fuji, but such infernal devices have been long regulated away here.

It is quite possible after much use on 60 cycle, the 50 cycle tools will have to be discarded and replaced either with 60 cycle ones, or used with a 50 cycle generator.

Thanks to you both, I spoke to a local electrician and his suggestion was to look at the colored wires in my female receptacle and expect to see 4 wires with color code as follows, red : Active, black : neutral, green " Earth, and blue or white : Common. He then said to replace my 4 pin receptacle with their 3 pin which looks like a smilie face with the two eyes as slits angled out ward from top to bottom at 45 degrees and the mouth a vertical slit on the center line. The faces right eye would be red : Active, the left eye Black : neutral, and the mouth green : earth, he said to cover over and eliminate the blue/white : common and everything would work fine. He had no problem with running their 240v / 50 hertz electric tools on our 60 cycles. I have been told so many false things here by nice people who only want to please me in the moment, and who will be very apologetic when I come back dissatisfied that I have to get confirmation outside of Fiji before I will trust this info.

In the US, 4 wire 240 cables will have a red and black hot, white neutral, and green ground. The red and black will be 240 to each other and 120 to the white.

As for the tools, you probably have the common sense to discontinue using them if they heat up, without us saying so.

In the US, the standard colors are:
hot = black
neutral = white
ground = green
2nd hot for 240v = red (tape both ends with colored electrical tape if using white wire from 2 wire cable - plain white is ALWAYS neutral)

Because the US 240 volt system works differently, you will want to connect one of the slanted prongs to black and the other to red (coming off the US colors that should be on the generator) and the green to the ground pin. The US neutral wire will be unused and should be capped off so it doesn't short to anything.