I think you may have to consider if it's definitely another child (teen/adult) you want, or if you have "empty cot" syndrome.... would having another baby keep you younger? Are you scared of facing the end of your fertility and having another baby will stave off those fears for another couple of years? Do you just want to keep going until the menopause? Would child no. 5 definitely be the end, or if you were say, 46, would you be tempted to give it another shot for the "last baby" then? If not, why not, and why do those reasons not apply now?
Also, and I am only suggesting this as a single-gender mum myself, and I realise that it may never have crossed your mind, but I have to raise the question, do you harbour any hopes, either consciously or subconsciously, that the next baby would be a girl, if you have 4 boys already? I know any child is a blessing, but is that a factor, even a small one, in your desire for another baby? If it does have any influence on your desire, how would you feel (apart from delighted, of course) if you got boy no.5 and still never had a girl? Obviously, like I say, if this has never crossed your mind then please disregard. I just feel it's useful to examine all the possibilities.
I have always felt well and young but now I am in my early 40's I am really starting to feel twinges of age. Things ache a bit more, I'm more tired, I have a bit less energy etc. I still have small children so that's no surprise, because they need so much that it is exhausting (brilliant of course too). I know you've done it 4 times already 
but can you really remember the gruelling getting up in the night, two or three times, every night? DC1 was hard work in the day but a good sleeper. DC2 is/was quite good in the day but a terrrrible sleeper. We were virtually begging for the broken sleep to stop. There were nights we were at breaking point with it. Even thinking about it makes me wince, it was a form of torture and it put a bad strain on me and DH. Can you deal with that again, and the nappies, and the drudgery, and the tediousness of toddler, spoon feeding and wiping noses and wiping bottoms and wiping fingers a billion times a day? 
Your youngest is now 5 and they can be surprisingly independant at that age.
The last thing I would ask (again, it may not have any relevance)... is there a trigger for this desire? Is there someone in your family (niece/sister/SIL/cousin) who is having a new baby, or even one of your friends, that has made you feel "I want to do that" because, after 4 children, you feel a bit of an expert on having babies but as your youngest is 5, you feel you are losing your grip on that as newer mums appear in your circle of family and friends? Again, I am only asking to be helpful, to help you make an honest decision, I am not supposing or thinking you are any of these things (so please don't take offence). Just tossing ideas around.