Welcome to HCM
We understand your concerns
I went through your details and am sure my answer will be upto your level of satisfaction.
Small penis is not a problem at all. For your information, almost all people have a penis size of 1 to 1 1/2 inch penis in flaccid state. The erect penis range from 3 to 8 inches.
Female vagina has pleasure neural ending only upto 2 inches from the surface. As your penis size is 4 inches erect, you should be able to satisfy any female and get satisfied yourself.
As far as random erections are concerned, it should be hormonal
activity unless you are actively desiring girls on sight and doing it. You can try masturbating daily for a week or so to reduce such random erections. Try pillow trick to masturbate.
I sincerely think you don't have any physical problems except the ones I mentioned above. Therefore don't unnecessarily worry.
Hope I answered your question upto your satisfaction. I am available for further queries.