I have successfully installed Gpg4win for my outlook client for mail encryption with one of my contacts. My private key works well with their public key. I was wondering if i can import my private key into my smartphone (android and/or iphone) so that I can decrypt these emails using active sync/exchange?
There are several EMail Clients that support encryption and decryption, like K-9 Mail or APG for Android or iPGMail or oPenGP for iPhone. Sidenote: The two mentioned apps for android are for free, but all offered apps for the iPhone are non-free.
To Sync your mails, use the IMAP settings of your provider, this will leave the mails on the server, so you can read them on any device.
Please also note, that the storage of your private key on a mobile device can be dangerous. Be very careful when storing them on your phone.
let me add that most mobile clients do not know about OpenPGP/MIME.
K-9 Mail will probably grow OpenPGP/MIME support within the next 9 month
and uses OpenKeychain (both available gratis as Free Software e.g. from fdroid.org).