Welcome to HCM,
Your penis size about 4 inches while erection is quite proper.
There is no problem satisfy your partner as in female initial 3 inch part of her vagina is sensitive so you can very well satisfy her sex.
2, Size of penis genitically built up so nothing can be done to increase size.
In normal condition penis size is always small so do not feel inferiority complex
and remove false fear from your mind and enjoy life.
Ok and bye.