Wow, was not expecting to come back here to find 6 pages of replies.
Firstly, thank you to all those who have been helpful!
To those who take this a bit too seriously, the ones who reply with comments such as 'A four string guitar is not a bass, of course you can't learn bass on something that isn't a bass, etc', I am aware a four string guitar isn't a bass, that is why I'm asking.
To those who think my question is stupid or trolling, well, there seems to be quite a few people who started off learning Bass on four-stringed guitars so I don't think it's as 'out there' as you seem to think.
But, as to those who have been helpful and haven't taken unnecesary offense at my question, I think you very much for your assistance and, if I find the time, I may give it a go! Thanks!