1040NR completion for Roth IRA conversion. 1099-R v 1042-S

Hi all.
This is my first time having to file a 1040NR to pay tax on a Rollover IRA -> Roth IRA conversion as a UK citizen/resident, non-US person.
I started filling out my 1040NR using OLT.com on the assumption I would be receiving a 1099-R from my Broker (Schwab Int.) reporting the Roth conversion. However, it turns out it will be reported on a 1042-S instead due to my Foreign status.
Is there a material difference in how I should complete the 1040NR as reported on a 1042-S instead of a 1099-R? I know there are different forms to complete within OLT.com but those do not form part of the filing package, so I'm really talking about the actual 1040NR form itself
e.g. Will the Distribution amount still be input into box 4a and the Taxable amount into box 4b? 
I will also be completing Form 8606
Any other considerations I should be aware of?
Many thanks.
Tim

I don’t know the answer but I would think that you would report it on line 4 since it is income from a retirement account, even though the withholding is reported on a 1042-S. (Maybe create a substitute 1099-R which is what I do with my UK pensions). An alternative would be to enter it as “Other income” but there is the issue of creating an 8606 to keep track of your ongoing basis.

https://trolley.com/learning-center/form-1042-s-what-happens-after-its-filed/

I don’t know the answer but I would think that you would report it on line 4 since it is income from a retirement account, even though the withholding is reported on a 1042-S. (Maybe create a substitute 1099-R which is what I do with my UK pensions).

Thanks Durhamlad - that's my default position unless I get compelling alternative advice from someone.

Maybe create a substitute 1099-R which is what I do with my UK pensions

I've now received the 1042-S but adding the 1042-S info into my 1040NR using OLT.com is problematic as it forces the Roth conversion amount to be tax exempt by Treaty as if it were a Distribution rather than a conversion and thereby creates further issues, so I'll have to proceed with creating a substitute 1099-R. I assume that as long as the IRS get the correct amount of Tax they won't mind.

I've now received the 1042-S but adding the 1042-S info into my 1040NR using OLT.com is problematic as it forces the Roth conversion amount to be tax exempt by Treaty as if it were a Distribution rather than a conversion and thereby creates further issues, so I'll have to proceed with creating a substitute 1099-R. I assume that as long as the IRS get the correct amount of Tax they won't mind.

I am not a tax expert or professional, and I am a dual US/UK citizen living in the UK so I am not speaking from actual experience here (you are a UK citizen living in the UK). However, since you have actually done a Roth conversion and need to also file an 8606 to keep track of the basis of your IRA then I feel that completing a substitute 1099-R should be acceptable. My wife (also a dual citizen) and I have each done lump sum IRA to Roth conversions which have only been taxed by the IRS so unless there is something specific about UKCs doing lump sum conversions I would think you are okay but maybe get another opinion or two.