I would recommend getting faster RAM, like this one:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820233265
Possibly you might want a SSD or a faster HDD. They will have smaller capacities though... Also, I just recommend these, if you need more space, keep your 4TB hard drive. I'm not sure if your build supports multiple drives, you could get an SSD and a HDD like the example below:
SSD: Samsung 840 EVO 120GB WD20EZRX (http://www.newegg.com/Product/Product.aspx?Item=N82E16820147247)
2TB HDD options:
Western Digital: Western Digital WD Green (http://www.newegg.com/Product/Product.aspx?Item=N82E16822236404)
Toshiba: TOSHIBA DT01ACA200 (http://www.newegg.com/Product/Product.aspx?Item=N82E16822149407)
Seagate: Seagate Barracuda ST2000DM001(http://www.newegg.com/Product/Product.aspx?Item=N82E16822148834)
This does add $30 to your original price. Also, I am in the U.S so I'm not sure if these items are available in the U.K.