It can be hard to find an all-threaded connection when it comes to barlows. The easiest solution would be to use the visual back and place the 1.25" barlow in that and follow that with a 1.25"-to-T-thread adapter and then on to your Nikon with its own T-thread adapter.
So, something like the following:
<Celestron Visual Back> <1.25" barlow> < 1.25"-to-T-thread adapter> < T-thread adapter for the Nikon> <Nikon camera mount>
And these would mate as follows: female(Visual Back slip-fit output) male(1.25" balow slip-fit input) female(1.25" barlow slip-fit output) male (1.25"-to-T-thread adapter slip-fit input) male(1.25"-to-T-thread adapter output threads) female(Nikon T-thread adapter input threads).
Other than the 1.25" barlow the only part you will need is: https://agenaastro.c...-t-adapter.html
Actually, on the 4SE it would be easier just to place the barlow in the right-angle viewing port as a "visual back" really isn't part of the standard 4SE configuration. So, it would be as shown above once the barlow is inserted into the standard 1.25" eyepiece holder.
Or, I guess you could add a 1.25" eyepiece holder to the 4SE's photographic port and avoid going through the flip mirror, but that would cost more than just using the existing eyepiece port.
Edited by james7ca, 19 January 2020 - 05:48 AM.