In terms of recycling, your council should have household recycling centres. These take extra items that can't be collected at the kerb, including electrical items. So, if these computers belong to you personally then that's the simplest option.
If these computers belong to your company then you should look at WEEE disposal, i.e. talk to another company who will take them away and give you a certificate afterwards. However, there's probably going to be a fee attached to that.
In practical terms, you could take a company computer to the household recycling centre. If there's only 4 in total, they won't care; they might ask for proof that you live in the area (e.g. a council tax bill), but they won't ask for proof of ownership. So, it would only be a problem if your company got audited and the auditor asks what happened to the old PCs that your company paid for; I think you'd be unlucky for that to happen!