When talking about "audio formats" it is important to make the difference between codecs and containers. M4A is a container format. A container is a method of storing audio streams in a file. Per se, it does not say anything about what compression has been used to encode the audio stream. In particular, M4A is popularly used for both AAC (lossy) and Apple's ALE/ALAC (lossless) codecs. So just going by the filename, you can't tell whether the audio has been stored lossy or losslessly.
What I can say with 100% certainty though is that if your files are even within the broader region of the size of an MP3 file, then they are lossy. The compression you get with FLAC is near-optimal. There is no alternative codec (with sensible encoding and decoding times) that will give significantly better compression than FLAC, and it's not likely to ever happen.
If you want your files in lossless quality, it is up to you what you decide to do. I keep everything in FLAC since it has always been an open format (that is also supported by my portable player), while ALAC is used practically only by Apple products. Compared to those two, all other lossless codecs are far less widely supported. The good thing about lossless though: if you ever think you made the wrong choice, you can always convert your library without any compromise in quality.
You can also check out Wikipedia's articles on the MPEG-4 Part 14
(that's M4A) container format, and the Advanced Audio Coding
(AAC) or Apple Lossless
(ALE/ALAC) codecs respectively.