Claim back VAT on Purchases before registration, What's the catch?

Hey Everyone!

I just heard that you can claim back VAT on Purchases before registration of up to 4 years given.

1. They are still used to make profits for your business today.
2. Have a VAT invoice for the item.
3. As long as you make a VAT sale with the assets still in use, you could claim back VAT.

Going VAT is a no-brainer to me.

1. I work almost only for VAT-registered businesses so charging VAT is not a problem.
2. I still intend to re-invest large portions of my profits straight back into the business.
3. Over the last 4 years, I Re-invested a great amount back into the business of which is still used today to make profits (Would have been great to claim back VAT the last year)
4. I also have the accounting software/services in place to deal with VAT.

But what's the catch?

1. Sounds like a great way to maybe save some profits for all the assets bought for the bussiness.
2. It feels odd to claim back potentially 4 years worth of VAT receipts but have not charged VAT to clients.
3. I need to check if I can even get VAT Receipts for most of my main purchases ect ect (A lot was through ebay)

What are other things I might be completely missing out here that I need to seriously consider?

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