Excellent product, looks just like the other 3 phones I bought in the Panasonic handset package. I bought this from Ligo who delivered speedily and the item was in mint condition, as expected.
However, adding the additional phone to the already connected 3 phones isn't intuitive so I'm including a brief guide in setting this up:
Hold down the intercom button on the main base phone for at least 5 seconds. "In Use" will show up on the main phone, but on the additional phone select "Handset setup", then "Register Handset".
This will take you to "Enter the BASE PIN" and you type in "0000" then OK. You should hear a small beep and the phone's original blank face will turn to the same screen as the other 3 phones.
Took me a while to figure that out but Ligo have decent customer service who aim to get back to you within 2 hours and so got me going sooner rather than later.
Great phone, great addition as it means my parents now don't have to rush to the kitchen when the phone rings as they have an extra phone in the lounge, one in their bedroom, and one at the top floor too. Great product!