Do not panic! I have been there; and have simply come to the conclusion that it is not a smooth run back to full health and mobility and that I have to resign myself to bad days mixed in with the good. I got very depressed at one stage, and was told that it is possible to become depressed after surgery - a combination of the general trauma, blood loss, pain, assorted drugs etc.
But this too will pass. My physio tells me that some people take a year to fully recover from hip surgery - they do not tell you this beforehand! - but that everyone gets there in the end.
They said that the up-and-down nature of the recovery was sometimes related to new bits of healing going on; and inflammation irritating other bits.
The most annoying thing was seeing friends who bounced back in a trice from their surgery - I am pleased for them, but,being only human, it is hard to swallow.
Really 4 weeks is nothing and there will be days when progress seems slow - or even backwards - but you just have to hang on in there.
I do feel for you - don't despair.
- I've previewed this message twice and the flowers do not appear - sorry