Not 4/5 months OP! I've learned this the very hard way unfortunately and feel immense guilt about it. Wait as long as you can OP. You can't know the man really for at least a year. and probably longer.
I've learned from bitter experience here - I had a bf who I thought was lovely and because I loved him, and didn't get much time away from the kids, I did introduce them probably after 3 months of knowing him. In hindsight he was a very charming narcissist, possibly a pyschopath, but I was completely reeled in. In fact, it was a very abusive relationship but I missed all warning signs (even when my 17 yo dd told me he was controlling, even when my friends warned me) - of which were lots ( in hindsight) and unfortunately it escalated to violence (luckily not in front of them) and I had to involve the police who involved social services.
The police were shocked at his actions- and social services told me that they would need to be involved if I got involved with him ever again (they said that if my children had been his children, they would have had to stay involved with me long term). Luckily my children didn't see the violence or the worse of the behaviour , but they did witness me in the aftermath of it and the relationship - really not good (ptsd, weight loss, insomnia, anxiety) - and of course (and to my eternal guilt), they did witness the controlling, abusive stuff before. I will not put them through that again.
So, I think a year probably and then make sure you do Claire's law checks and heed any warning signs at all.
I know that's an extreme situation, but I really don't think you can know a person within 4/5 months and it probably takes much longer than a year to even scratch the surface.
I do feel guilty about it, although my children seem fine (and joke about him now - they still don't know about the violence) but have def. learned my lesson.