The address to send the forms to is on the top of the ATF form 4. I normally order ATF forms from this website; http://www.atf.gov/forms/firearms/
The ATF form 4 is not on it now, it is probably being revised, again. You can order the forms here free of charge. http://www.atf.gov/forms/dcof/
You will also need to order two fingerprint cards. Form 5330.20 is here. http://www.atf.gov/forms/download/atf-f-5330-20.pdf
If you live in AL, AR, AK, AZ, CO, CT, FL, GA, ID, IN, KS, KY, LA, ME, MD, MO (with C&R), MS, MT, NE, NV, NH, NM, NC, ND, OH, OK, OR, PA, SC, SD, TN, TX, UT, VA, WA, WV, WI or WY and can pass the NICS background check, then you can expect the ATF to approve any AFT form 4 you send in as long as it is filled out correctly. Check your local laws, some state require registration, in TX silencers are technically illegal but registration is an affirmative defence to prosecution (but not arrest). I read that WI requires that the local sheriff inspect your home before he signs, but I have not actually looked up the law on this yet. Sounds like BS to me, but is it best to read the law first. Asking your local sheriff is probably not a good idea as some of them are just going to tell you that silencers are illegal in the USA.
If the tax stamp application is not approved for any reason, the ATF will refund your money. As far as I know they are not allowed to keep money sent in for stamps not issued. I can post a link to the law if you need it.
Many people use a trust to own their title 2 firearms. This eliminates the requirement for the sheriff's signature and the fingerprint cards. I used Quicken Willmaker to crate my trust. Some people spend $300-600 bucks to get a lawyer to print one out for them. You can even used a shared trust so that more than one person (the trustees) can own the silencer even if theya re in different states.
Some people (like me) make their own silencers on the ATF form 1, same $200 tax, but only about $20-60 in material costs. But you need access to a lathe to make a decent silencer. It takes the ATF 2-6 months to approve my forms.