Just a few comments on the subject
FileMaker is certainly cheaper than some enterprise solutions in licensing costs. However, the real cost benefit is in development time. The development life-cycle is typically orders of magnitude lower than other enterprise platforms (whatever the licensing costs of those platforms). By this I mean days instead of weeks, or weeks rather than months to develop some feature.
There is a strong argument that FileMaker is Access for the Mac. While this was a valid argument a few years ago, FileMaker has come into its own in recent years. It's worth noting that FileMaker is cross platform and used extensively on Windows as well as Mac. That being said there are still huge similarities and differences between FileMaker and Access, the truth is none of them have any bearing on your situation.
While FileMaker is non-standard it does support live connection to MySQL, MS SQL Server and Oracle.
Also, there are numerous FileMaker developers not as much as more standard platforms, but they are definitely about, if you let me know where you are I can put you in touch with a selection of developers in your area.
The important point I want to make is that in the correct context FileMaker is the best thing in the world at what it does - if you try to do something that it's not meant to do, you'll get stuck. However, it could support offices in 4 locations, it can and is being done.
Before you go and rewrite your system in some other platform you should get in touch with a FileMaker expert and see what they have to say about what you've currently got, writing more details on this site and having non-experts answer positively or negatively won't help you. In the end it has to be a business choice of costs vs. benefits.