Please read first Getting the most out of this forum
This is not the same situation and you should create new post.
What is “10.10.4.0/24”? You say your LAN is 10.10.10.10. Please read your post in Preview before submit it.
About your case.
At MikroTik who is after other router like FortiGate must have a directly connected network with it. I understand that “Wan interface ip 10.10.9.2” is on MikroTik and 10.10.9.1 is on FortiGate.
When you not use SNAT/Masq NAT rule then you used a routing itself, this mean your LAN 10.10.10.10 or/and 10.10.4.0/24 is out to FG as original address.
This FortiGate must have do a SNAT rule to hide your 10.10.10.10 or/and 10.10.4.0/24 as 10.100.20.0/24 because this 10.100.20.0/24 is internal network inside FortiGate, this is not MikroTik network.
You cannot do SNAT (hide src address) as not your network. SNAT can change incomming not-your network as one of yours networks. Means you can hide 10.10.10.10 and/or 10.10.4.0/24 as 10.10.9.2. When you do it then still FG must do SNAT to hide your 10.10.9.2 as 10.100.20.0/24.
If I not answer you properly that means you wrongly describe problem, not use a diagram. Please read first Getting the most out of this forum