Hmph - what a question!
I have an SP 4 - was my 1st plane, still got it, still use it. So, what do you need to know about it?
1) it's a modern record, so the casting's not too hot - may not be flat soled, sides may not be 90 degree to sole.
2) the iron's worth replacing
3) the SP mark means it's the cheap end of the Record range
4) the handles are nasty plastic, but they can be replaced easily
5) the frog has no bailey or bedrock style adjustment...record say the frog isn't adjustable.
So, all the above sound bad, but, it's only 25-30 quid, it works, and contrary to record advice, you can adjust the frog, albeit slightly. Jeff Gorman reckons he can fettle an SP4 into a decent smoother, and I would agree with him - mine is still a good plane, though it's been supplanted by a vintage stanley 4 1/2, bought for 18 notes from a car boot - even better value.