Excess (very high) levels of prolactin can have a negative impact on libido.
Prolactin <425 µg/L (20 ng/mL) in most cases would not be a cause for concern.
With a trough TT 791 ng/dL and lowish SHBG 19 nmol/L, you should have a high-end FT.
Even then the only way to know where your FT level truly sits is to have it tested using the most accurate assays such as the gold standard Equilibrium Dialysis or Ultrafiltration.
Your SHBG barely budged.....22-19 nmol/L.
Again you just started trt and you are only 4 weeks in.
It is common for many to experience what we call the honeymoon period (euphoric feeling/strong increased libido and erections) during the transition due to rising androgen levels/increase in dopamine and lighting up of the AR (androgen receptors).
Unfortunately, this is usually temporary and short-lived as the body will eventually adapt.
Libido let alone erectile functions are multifactorial and much more is involved than having healthy hormones.
When looking at the bigger picture the first 6 weeks whether starting trt or tweaking a protocol (dose T/injection frequency) means nothing.
You need to give the protocol a fighting chance.....at least 12 weeks to gauge how you truly feel regarding relief/improvement of low-t symptoms!
As I have stated numerous times on the forum:
Achieving a healthy TT/FT level let alone other hormones on such protocol should have a positive impact on (mood/energy/libido/erections/recovery) but it will be more in the norm as opposed to the so-called amazing!
You need to get out of the mindset of trying to chase this honeymoon period as it is temporary and short-lived.
Shut down of the HPTA most likely has a big impact.
Hate to burst your bubble if you think that having healthy testosterone levels let alone constantly fluctuating will result in a raging libido/killer erections.
There is so much more involved when it comes to libido/erectile function.
Having healthy testosterone levels is one piece of the puzzle.
Even then when it comes to trt and libido many tend to get caught up in thinking that it will be through the roof once they hop on trt and get to the point of so-called dialed in let alone cure any ED issues they may have.
When you find that happy place you should have a healthy libido not raging, savage, insane.
There are many men who will see an improvement in libido, others will continue to struggle, some may even end up worse off than before trt and some of the lucky ones will see a drastic improvement.
I think too many get caught up in expecting to feel great 24/7 once on trt as if testosterone is going to cure all that ails them.