looking for 4 Axis Free CAM software on Linux

I don't think that there is any. 4-axis CAM is difficult, and folk are only likely to throw the required resources at the problem if there is hope of getting the investment back.

I might be wrong, though.

Depending on what you are trying to make, have you considered hand-coded G-Code? With the loop and branch structures in EMC2 G-Code you might find it easier than you think. This is a 4-axis program to cut sprockets, for example:

; Sprocket
; Assumes (0,0) is at the centre of the rotary table.

;Input Data
#<Pitch> = 20
#<Teeth> = 16
#<Roller_Dia> = 10
#<Cutter_Dia> = 8
#<Thickness> = 3
#<Feed> = 30

;Some calculations
#<Step> = [360 / #<Teeth>]
#<H_Pitch> = [#<Pitch> / 2] ; Half Pitch
#<Pin_Off> = [[#<Roller_Dia> - #<Cutter_Dia>]/2]
#<H_Width> = [#<H_Pitch> - #<Pin_Off>] ; Half Tooth Width
#<Radius> = [#<H_Pitch> / tan[#<Step>/2]]
#<Tooth_Height> = [SQRT[[2 * #<H_Pitch> * #<H_Width>] + [#<H_Width> *
#<H_Width>]]]

#<Angle> = 0
G0 A#<Angle>
;Spiral down
G0 X#<H_Width> Y#<Radius> Z .1
G2 Z[0 - #<Thickness>] I#<Pin_Off> F#<Feed>

;Machine Teeth
O100 DO
G3 X0 Y[#<Radius> + #<Tooth_Height>] I[#<Pin_Off> - #<Pitch>]
G3 X[0-#<H_Width>] Y#<Radius> I#<H_Pitch> J[0-#<Tooth_Height>]
G2 I[0-#<Pin_Off>]

;Move Table round a step, leave cutter parked in the hole to show off.
G0 X[0 - #<Pitch>/2]
#<Angle> = [#<Angle> + #<Step>]
G0 A[#<Angle> - #<Step>/2] X0 Y[#<Radius>/COS[#<Step>/2]]
G0 A#<Angle> X#<H_Width> Y#<Radius>

O100 WHILE [#<Angle> LT 360]

M2