Unless you are trying to breed/not breed it doesn't matter that much unless you are getting another conure and planning to house them together. If you are planning to get another conure of a different species you would do best with a same sex bird since most conures can hybridize. You would be best getting a bird somewhat similar in size if you intend for them to interact.

You do have to consider, though - if he's been an 'only bird' for 4 years, he may not welcome another bird. He may think of himself as a person and just see another bird as competition for your attention. Another bird will help keep him company when you're gone but may take away from the time you can spend with him when you are home.
Another possibility is adding a pair of finches or a canary in a large flight cage. They don't need much attnetion or time out of the cage, and would help keep him company.

Karen
Chico (conure) and Pippin (budgie)
WI

If you do decide to get a second bird make sure you have the new one vet checked and plan on keeping them seperate for a while you need to make sure the new bird does not pass anything on to the bird you already have. Also you cannot just go out and get a new bird and put them together. They have to be introduced slowly and supervised to ensure they are not fighting. Once you are sure they are getting along and will not fight you can start trying them in the same cage, but still only while supervised, Birds can be very cage aggressive and possessive and the older Bird may feel the need to defend his home. I have five Macaws and they all have cages and we have a few playstands. I can move them on to eachothers cages but I never leave them alone because they will go after eachother if they are in a mood. They do not mind sharing the playstands and can even go on them together but they all have there spots and if someone is not where they belong then watch out.

Taking on a second bird is a big commitment. You could end up with two birds demanding an already limited amount of time instead of just one.

They do make DVD's and CD's for parrots to keep them company and teach them at the same time.

This might be a better option at first, also make sure to have several toys and alternate them regularly. Place one or two in the cage and a week later swap them out for two different ones, then keep rotating them. Your bird will be less likely to bore of the toys and be more likely to occupy himself this way.

I hope this helps.