By law, you need to let HMRC know you are trading, as a business, within three months of starting up. I suggest you do so sharpish, as you mentioned its been four months so far. You can fill out the form online, take a look at the HMRC website.
VAT registration does mean you can claim back VAT, correct. However, you will now need to charge your customers VAT once registered. I personally wouldn't register unless you need to (i.e. if your turnover is over £64,000).
Its best to do things by the book from the start, HMRC don't take kindly to those trading illegitamately. Its better to be safe, than sorry.