400M seems huge for size of transaction log although it's all relative to how active your site is and size of the database. First thing to do is check if you have transactions which all still open. Transactions should be opened and closed within seconds. Opened transactions will let the transaction log backup truncate up to the first opened transaction. There's a system stored procedure which you can run to get info about opened transactions. I can't remember the name of it. Easy to Google it.

Second.. you should have log backups running several times a day. How frequent depends on the activity of your database. Activity includes deletes, updates and inserts. These are logged. You should ask your host or server admin and see if there are any scheduled jobs running to do log backups. If not, ask them to set one up. If you switch to simple mode, you will lose all the transactions since the last full or deferential backup if you ever need to restore your database after a crash.. If that's not important for you, simple is fine. However if you want to be able to restore everything up to the point of database crash, keep the full recovery model and investigate why the transaction log is big. The points I mentioned will help you in that direction. Instead of working around a problem, find the cause of the problem.

Also interesting to note I have 2 sites with huge log files, 1.3 gig and 1.5 gig respectively. One of these sites hasn't even been released to the public yet, they both run DNN 7.2, the other sites on this server are on different versions and have much smaller transaction log files. Don't know if that means anything top anyone but thought I'd throw it in.