Thanks for your question and concern. 4 ml is the average fluid amount discharged by a man during his ejaculation and a normal condom is capable of holding this much amount of semen in it. But some men will ejaculate more than this amount and while some others ejaculate less amount so it will be good in trying different condoms. The condoms usually come in different shapes and sizes so you can find an appropriate condom that may suit your boyfriend. You could also try female condoms which are usually large in size than that of male condoms.
The erection ring and other type of products such as delay lubricants and delay condoms usually help in slowing the ejaculation and sometimes prevent the ejaculation too. If you would like to use these types of products then you may try and see if they can be helpful. If these are the things that make you and your boyfriend concern then it will be helpful if you would talk to a doctor or to a nurse.
The important thing to keep in mind is about the sexual health. Do not endanger your sexual health under any circumstances. The sexually transmitted diseases can be easily passed from one person to another person by simply through the sexual fluids. The sexual fluids usually include the semen and the vaginal fluid. But most of the infections are easily treatable. So if you are worried about sexually transmitted infections then it might be good if you get it tested and thus put your mind at ease. You can easy testing at GUM (genitourinary medicine clinic). The consultants at the GUM clinics are free and are confidential. They will give you free advice and also diagnosis the infections properly. Take care.