Set decimal places - Texas Instruments TI-84 Plus Calculator

Question edited for clarity. It is 'decimal' not 'decibel' which is a measurement of noise levels.

It will show up to 9 decimal places. Page 3 of the manual refers

https://uwm.edu/libraries/wp-content/uploads/sites/59/2021/04/fx-300ESPLUS_2ndEdition_QSG.pdf

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You can select the A (K . A) and then the # of decimal places (F 6 3 2 1 0) you want. So if you want to enter 1234 & it calculates 12.34, this will work.

The selector you are asking about is the "decimal selector". The result displayed/printed, will be in a format according to the position that you have selected as follows:
f = floating (the result will be in the number of decimal places that were entered on the keyboard
4= fixed 4 decimal places (x.0000)
3= fixed 3 decimal places (x.000)
2= fixed 2 decimal places (x.00)
0= fixed 0 decimal places (x)
add 2= (add mode*,fixed 2 places) ($.00)
Note* "ad mode" automatically enters the decimal place when entering the number on the keyboard. If you enter "123", the displayed ammount will be "1.23". If you enter "1", the display will read ".01". This is a useful setting when adding a list of numbers in a dollars/cents format, however, be careful, if entering a whole dollar ammount "three dollars" be sure to enter the decimal point, or enter the number as "300". If you don't, the result will be ".03", instead of the 3 dollars you intended.

CUT: Values are cut off to the number of decimal places speicified by the current Decimal Place Selector setting

UP: Values are rounded up to the number of decimal places speicified by the current Decimal Place Selector setting

5/4: Values are rounded off (0,1,2,3,4 are cut off; 5,6,7,8,9 are rounded up) to the number of decimal places speicified by the current Decimal Place Selector setting

Decimal Point and Separator Symbols
You can use the display setup (Disp) screen to specify the symbols you want for the decimal point and 3-digit separator.
• To change the decimal point and separator symbol setting, press the MODE key a number of times until you reach the setup screen shown below.
Display the selection screen. (Disp) 1.
Press the number key ( 1 or 2) that corresponds to the setting you want to use.
1(Dot): Period decimal point, comma separator
2(Comma): Comma decimal point, period separator

Do you mean four places behind the decimal point? Page 21 describes setting for fixed decimal point 0 thru 9 places to the right.
http://education.ti.com/downloads/guidebooks/graphing/83p/83m$book-eng.pdf

Your BA II PLUS stores numeric values internally to an accuracy of 13 digits. However, the numbers it displays depends on the number of decimal places to which you have it set (choice of 0–9 or floating decimal). To set the number of decimal places displayed to three: hit [2ND][FORMAT][3][ENTER]This setting also continues indefinitely (even though the calculator is turned off and on) until it is changed.

Your BA II PLUS stores numeric values internally to an accuracy of 13 digits. However, the numbers it displays depends on the number of decimal places to which you have it set (choice of 0–9 or floating decimal). To set the number of decimal places displayed to three: hit [2ND][FORMAT][5][ENTER]This setting also continues indefinitely (even though the calculator is turned off and on) until it is changed.

I found this solution on the HP website. Turn it on, and do what it says. Then turn it off and back on again, and the extra decimal places should show.

http://h10025.www1.hp.com/ewfrf/wc/document?dlc=en&lc=en&cc=us&docname=bpia5183 Introduction This document describes how to set the desired number of decimal places on the HP 12C Calculator in standard format and in scientific notation. Instructions for standard format

1. Press and release the gold PREFIX key (f).
2. Press the number key for the desired number of decimal places (0 through 9).

The display will show the specified number of decimal places. The answer will be rounded to the number of digits selected.

2ND [FIX] 4