@ConcernedUser 

See this forum topic conversation for M9: Deco M9 Plus power adapter barrel plug size.

I suggest you rephrase your question same way and post it on that forum, but for S4. When you get answer from TP-Link, you could get extension cable from places such as Amazon, with the right specs.

I mean, extension cable like this one (image taken from Amazon):

EDIT: Someone have asked that question already, see Deco S4 power adapter barrel plug size

@Alexandre. Thank you very much

I was looking for an extension cable, but didn't know what size adapter I would need or anything of the sort. The links you've posted are helpful. Is the Deco S4 definitely 2.1mm plug? Do you think this would be OK?

https://www.ebay.co.uk/itm/DC-Power-Supply-Extension-Cable-12V-for-CCTV-Camera-DVR-PSU-Lead-1m-2m-3m-5m-10m/261870863580?

@Alexandre. Also do all Decos use the same power supply or is the M9 and S4 different?

@ConcernedUser 

Can't speak for S4, but power supplies for M9 and M5 I have in my house are definitely diffrent. It is obvious just by looking at them, see photo of M5 and M9 power supplies plugged to electric outlets side by side. M9 power supply is on top.

@Alexandre. Thx that's helpful

do you think the extension cable i linked above would work for the s4? just not sure if i got the right one/measurements. seems right tho

@ConcernedUser 

Any length cable which has measurements for the plug of 5.5x2.1mm will work. I have the S4 and I bought a 3m length for £2.99 on eBay. Works great.

The ebay listings change quickly and regularly on eBay but this advert looks similar to mine:

eBay Item Number: 313263120195

@Vanellus Thank you, very helpful