The most important thing is to not mix brands… they have very different sounds.
I think I would look at it this way. The HTM2 is a good substitute for the 4 until you can afford a 4. If you listen to two channel, then your left and right are the most important. You are distracted while watching video and the details of speakers are not as noticeable.
The HTM4S is a better sounding speaker than the 2. 85% of the content in HT comes from the center. You want better sound, then at some point, you can upgrade.