Opto-coupler connection
First, it's not ok to say "5V will reach base"?
If you means the output-side "+5V" reach the darlington's base, you're right.
If you means the left "5V"  reach the optocoupler's transistor base the answer is, obviously, not, because the optocoupler is used exactly to maintain disconnected input and output.

Second, Phototransistor in optocoupler is used in switch mode. When the light is "enough" the phototransistor saturate. Then the 5V in the output-side pass through the phototransistor, that act like switch, and reach the darlington Q1-Q4. 

You not need to work with \${V_{BE}}\$ because in this case we use the photovoltaic effect (there's a little bit of phisics we need to know).
Also, you don't need top put emitter to the ground, like said in comments, because without current the Darlington pull down the phototransistor's emitter.  

If you put a LED, as asked in comments, it work like normal BJT switch, like in this schematic (LTSpice file at bottom); you can see that the \$V_E\$ is zero while microcontroller (V1 in schematic) don't assert output.


You can read more about phototransistor in the first page of this Fairchild Application Note, in some optocopuler's datasheet and in the productor's knowledge base, like:


Toshiba
Vishay


LTSpice file:

Version 4
SHEET 1 880 680
WIRE 128 -240 112 -240
WIRE 144 -240 128 -240
WIRE 128 -80 128 -240
WIRE -96 -32 -112 -32
WIRE 64 -32 -96 -32
WIRE 240 16 240 -16
WIRE 240 16 128 16
WIRE -112 64 -112 -32
WIRE 240 64 240 16
WIRE 240 176 240 144
WIRE -352 256 -352 160
WIRE -112 256 -112 144
WIRE -112 256 -352 256
WIRE 240 256 240 240
WIRE 240 256 -112 256
WIRE 240 272 240 256
FLAG 240 272 0
FLAG 128 -240 +5V
FLAG -352 80 +5V
FLAG 240 16 Ve
FLAG -96 -32 Vin
SYMBOL diode 224 176 R0
SYMATTR InstName D1
SYMBOL res 224 48 R0
SYMATTR InstName R1
SYMATTR Value 1k
SYMBOL npn 64 -80 R0
SYMATTR InstName Q1
SYMBOL voltage -112 48 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value PULSE(0 5 0.5)
SYMBOL voltage -352 64 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value 5
TEXT -386 296 Left 2 !.tran 1