Please clarify your question...
1. Do you want to import an AVCHD.mts file into Premiere Elements 4?
2. Do you want to export a file from Premiere Elements 4 that has a .mts file extension?
3. Something else?
If you have an video file that uses AVCHD video compression, please be advised that Premiere Elements did not support AVCHD until Premiere Elements 7.
If you did want to use an AVCHD.mts file in Premiere Elements 4, one path might be to convert the AVCHD.mts to MPEG2 HD.mpg with one of free video converter. The free Koyote Soft HD converter was popular at one time for that conversion.
There are other converters that you could explore. At one time, if the Premiere Elements 4 user's computer was Sony Vaio, the user could get an Add On from Sony that would force Premiere Elements 4 to be compatible with AVCHD. You could always move on to another version of Premiere Elements that does support import, editing, and export of AVCHD.
Remember that AVCHD is a very resource demand format to edit. Will your computer resources be able to hold up those demands?
Please review, consider, explain, and then we can decide what next.