A kit of 8ch analogue DVR and 4 cameras is probably your cheapest and easiest option. Have you looked on eBay?
Aliexpress, banggood etc should also have lots of cheap options.
Here's one that includes four cameras to add to the four you already have. Excludes disk drive, so if you have one spare you save a bit, or add £40 for a drive.
Claims ONVIF IP Camera support as well.
Going for all- IP camera system could get more expensive than a composite video system. I've seen IP cameras for around £35 each. Eight of those is probably more than you wanted to spend to just add four cameras.
- 1 X 8CH 1080N HDMI CCTV AHD TVI DVR (Hard Drive Not Included)
- 4 X 1500TVL Waterproof IR-Cut Bullet Security Cameras
- 4 X 60ft(18.3M) Video + Power Cable for Security Cameras
- 1 X 1-4 Power Splitter Cable for Cameras
- 1 X USB Mouse For DVR
- 1 X Power Supply for DVR (UK Plug)
- 1 X Power Supply for Cameras (UK Plug)
- 1 X User English Manual
- 1 X CD Software
https://www.ebay.co.uk/p/Floureon-8-Cha ... 3103234450
I am not recommending this product. I only now what the eBay listing claims, but it seems like it might meet your needs.