At the risk of starting a 3rd multipage argument over this exact question this month... Here is a rule quote for difficult terrain:
Every foot of movement in difficult terrain costs 1 extra foot. This rule is true even if multiple things in a space count as difficult terrain.
So with that, movement would cost 4+1=5 to move 1 foot, and slow halves your speed which effects the total you can move, not the cost of movement so stacks separately.
Plant growth - 4ft of movement = 1 ft
Difficult terrain - 2ft of movement = 1ft
Slow - Movement speed is halved
Do these stack?
Difficult terrain is not listed as “halved” it’s listed as “spend 2ft if movement for every 1ft moved” Using wording just like plant growth.
I rule they do not stack, the most detrimental effect applies
At the risk of starting a 3rd multipage argument over this exact question this month... Here is a rule quote for difficult terrain:
So with that, movement would cost 4+1=5 to move 1 foot, and slow halves your speed which effects the total you can move, not the cost of movement so stacks separately.
See https://www.dndbeyond.com/forums/dungeons-dragons-discussion/rules-game-mechanics/84300-question-about-difficult-terrain which covers slow too.