Usually a knock means either your head bearings are loose/worn, or you need new fork bushings. The other one I had once was on a Triumph where the disc bobbins were loose, that had me foxed for a while.
Has the seller sold you bits that are a tiny bit duff, perhaps? Seems to me like they are missing dust caps, have had some sort of bodge on he fork seals and if my guess is right also want new bushings. It's not the end of them if they're straight and the chrome's okay but the 4 bushings will set you back ITRO £19 + VAT + P&P from David Silver:
http://www.davidsilverspares.co.uk/CBR4 ... art_11094/
http://www.davidsilverspares.co.uk/CBR4 ... art_11095/
Oil seals are £11 + VAT + post there, Dust seals £15.50 + VAT + post, but you can get reasonable quality Pyramid parts on eBay for ~£17 for all 4, delivered.
You can spot duff fork bushings by getting the front wheel off the floor, holding fork lowers tightly and wobbling, if you can see movement at all, they want changing. They often get neglected but they are definitely a consumable item and your bike will handle better for changing them - less play and less "stiction" as when they wear, it's the teflon coating which gives out first. Since you would put new oil in at the same time there is another small benefit to be had.