If you look into the compatibility Matrix: http://us.blackberry.com/content/dam...sept_20_12.pdf
it rather looks, that BES 4.1.x will not work with Exchange 2010. You could try if it works or not, but if it later turns out it does not, your 40 users will have a problem.
You should first upgrade to BES 5, then upgrade Exchange
BESx Express is free, but limited in its functionality regarding the amount of policies. I dont know, if the BESx Express is the right choice for you. Here: http://us.blackberry.com/apps-softwa..._NA_012110.pdf
is a simple comparison of the features.
You can find an upgrade from the Full BES 4.x to 5.x for about $1000. If you cannot google this for yourself, look for a company called Zezel. I bought several product from them, and I never had a reason to complain.