I connected 4 x 1000Ω resistors in parallel. Left side is 5V and right side is ground.   

1 1 1 1 1
- = - + - + - + -
R(total) R1 R2 R3 R4

1 1 1 1 1
- = - + - + - + -
R(total) 1k 1k 1k 1k

1 4
- = -
R(total) 1k

1 1
- = -
R(total) 250

R(total) = 250Ω

Just the four resistors in parallel are 250Ω

Here is with just the resistors

Here is with the Uno connected but no power.

Where is the extra resistance coming from?  The Uno.  It reads 3.03 kΩ

I will prove it mathimatically

 1               1               1
----    =    -----     +    -----
231         Runo                 250




1               250 + Runo
---      =      --------------
231            250 * Runo

250 * Runo   =  231 ( 250 + Runo )

57750 = (250 – 231) Runo  

57750
-----------    = Runo   =   3.039kΩ
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