Is there a way to practice changing amount in line 4a to view changes in tax liability can i use 2018 return planning rmd for 2019

Do NOT use your completed and filed 2018 tax return to make estimates for 2019.

Use the TurboTax calculator TaxCaster for estimates of a 2019 tax refund or taxes owed - https://turbotax.intuit.com/tax-tools/calculators/taxcaster/

Note that line 4a doesn't affect your tax anyway; line 4b does.

Besides, the calculation of line 4a is anything but obvious - some distributions are included there and some are not. You would go nuts trying to figure out why the amount in 4a is what it is.

So go use the TaxCaster as DoninGA suggests; it has been updated for 2019 tax laws as much as is possible at this point...