Hi,
Thanks for posting on the website...I am guessing that you aren't a physio.
First of all, it seems like the diagnosis given to you (as told to us by you) is a little simplistic. Sometimes this is because the physio doesn't want to "scare" you with something that sounds bad. I would personally ask if you have hurt something in your back. The reason why is that decreased motion at L4 and L5 is merely an observation and you haven't really given the reason why this has happened. I am sure your physio can give a good clear explanation why.
Secondly, there is plenty more you can be doing. I often give similar exercises to my patients during their first visit for a number of reasons - one of them is to see if they are "compliant". That is, if they can't do these simple exercises regularly, then they probably won't do harder ones either. Another reason why is to see how you respond to these simple exercises.
Thirdly, we can only go by what you tell us. Did the physio do anything else? Did he/she say anything else?
Fourth, it is hard to recommend you to do something like foam roller exercises without knowing what is wrong with you. The fact that you wrote a post like you did suggests to me you are quite keen and "searching" for answers. It might be because you don't trust or like what your physio has told you or it was not what you expected. Or it could be that the physio didn't explain it in a way that made sense to you. Whatever reason it is, you need to find someone who can help you and deal with you in a way that works best for you - the physio that you saw might be quite a good physio but not a good personality match - who knows?!?
Lastly, If it were my back and I couldn't sit properly for 4 months, i would spend the money required to get the best advice i could. Now it is easy for me to say that but i can appreciate that not all people can afford this. But if you think about it, can you afford NOT to do it? If this person can give you all the information you need to understand your problem and fix it quickly, you often don't need to see them often, especially if you do your exercises. I can often help people in less than 5 sessions. Even if you went for one or two sessions and told them upfront you need a diagnosis and treatment plan so a NHS physio can follow you up, then i would consider it. Especially if i lost money by NOT working. But you don't sound that bad.
I hope the above helps. There are so many things that you could have wrong with you like a disc injury, twisted pelvis or lower spine, sprained back joint, overactive muscles in the back, poor core stability, etc etc.
Good luck!