If these were payments to which you were entitled to at the time you may indeed try to pursue these now from the employer. The fact that this was supposed to be paid to you a few years ago does not really change anything and whether this was due last week or 4 years ago, the fact is that you are still legally entitled to be paid. Had you been the one owing the employer money they could have chased you for it now so it is only right that you can do the same.
The issue is that if they refuse to pay you, then you will have to consider your options on how to take this further. Your only option would be to make a claim for breach of contract in the county court. So you will have to take your current employer to court over this. This should not affect your employment with them because you should not be treated detrimentally for claiming against them but it may not make you the most popular person in the workplace. Still it is your legal right to pursue this if you have to and whilst I suggest you try and resolve this directly with them, if that does not work then you can resort to the legal route. You can make the claim online via www.moneyclim.gov.uk.
Hope this clarifies your position? If you could please let me know that would be great, thank you