@Zpmrk 

You can use a S7 as a main deco in an E4 network, or an M4, both have Gigabit ports.

Main differences are 

1. The S7 has 3 network ports and the M4 has 2

2. The S7 has faster Wifi (AC1900 vs AC1200 in the E4/M4)

I'd say the S7 is a more powerful device to use as the main router.

Do you have your other Deco devices connected by ethernet or Wi-Fi Mesh?

  @Zpmrk 

Zpmrk wrote

I have a total of 4 deco e4, 1 is the main node and connected to the router while the other 3 are "slaves" that are connected to the main node wireless. Only problem is I dont know if Deco S7 will work with them or if I use Deco S7 as the main node the 100mbps limit from the other E4s will be removed

100% the S7 will work with the E4's

The 100Mb limit with the E4 is just for the ethernet port, the wirless speed between the Deco's (depending on distance) is going to be between 300-500Mb/s

So basically what is happening now is, the ISP is sending 400Mb, when it gets to the Deco it drops it down to 100Mb and sends that out to the other Deco's at 100Mb/s because it was capped by the ethernet port at that.

If you replace the main deco with an S7 or M4 or M5 or M9 or even a newer X10, all of which have 1Gb ethernet ports, then what will happen is your ISP will send you 400Mb/s and the Deco will receive 400Mb/s and send 400Mb/s out to the other Deco's because there is no cap on the ethernet port.

Just get whatever is cheaper for you in your location (S7, M4, M5, M9, X10)

  @Zpmrk 

For Ethernet

So your ISP is sending out 400 Mbps to your modem, which then sends out 400 Mbps to your main deco, your main deco can only handle 100 Mbps so it sends all other deco's 100 Mbps.

If you replace just your main deco then your main deco will receive 400 Mbps but the other deco's can only receive 100 Mbps because they are only made to receive 100 Mbps

For WiFI

Any devices that connect to the S7 will have the ability to get up to the following speeds this depends on how fast your internet is from your ISP and a lot of other factors.

5 GHz: 1300 Mbps (802.11ac)
2.4 GHz: 600 Mbps (802.11n)

Any devices that connect to the E4 units will have the ability to get up to the following speeds this depends on how fast your internet is from your ISP and a lot of other factors.

5 GHz: 867 Mbps (802.11ac)
2.4 GHz: 300 Mbps (802.11n)

The four deco E4 units you have cover up to 6,000 square feet if you have the cash I would replace all 4 E4 units with a two pack of deco model XE75 Pro  

https://www.tp-link.com/us/deco-mesh-wifi/product-family/deco-xe75-pro/v1%20(2-pack)/

The two pack can cover up to 5,500 square, it has 1× 2.5 Gbps Port + 2× Gigabit Ports and much faster WiFi than the S7 as shown below 

6 GHz: 2402 Mbps (802.11ax, HE160)
5 GHz: 2402 Mbps (802.11ax, HE160)
2.4 GHz: 574 Mbps (802.11ax)

  @Zpmrk 

Since the E4 units can only push out 100 Mbps the WiFi will be limited to that on devices connected to the E4 units, however if most of the devices are in the range of the S7 you can force them to connect to the S7 VS the E4 units 

This post will give you a good example of what to expect for WiFi speeds with the devices that connect to the E4 units 

https://community.tp-link.com/en/business/forum/topic/670110

  @Zpmrk 

You're correct in your thinking. When using Ethernet backhaul, the Ethernet port speed on each Deco unit can become a bottleneck for the overall network performance. Since the Deco E4 units have 10/100 Mbps Ethernet ports, the backhaul connection speed for those units will be limited to 100 Mbps. This means that the total throughput available to all WiFi clients connected to those E4 units will be constrained by that 100 Mbps backhaul connection.

So while the E4's WiFi radios themselves may be capable of higher speeds in ideal conditions, the actual speed your devices can achieve when connected to those E4 units will be limited by the 100 Mbps Ethernet backhaul. This is similar to having a pipe that narrows at one point • the narrowest section determines the maximum flow.

According to our Ethernet backhaul guidelines, we recommend using CAT5E (or above) Ethernet cables between Deco units to achieve gigabit speeds, and ensuring that all network switches and ports involved also support gigabit speeds. In your mixed setup, the Deco S7 (which has gigabit ports) will be able to utilize faster backhaul speeds, but the E4 units will remain limited by their 100 Mbps ports.

  @HelpFixDecoApp 

Seeing as he doesnt have ethernet backhaul. He's said multiple times that the satellite E4s are connected via W-Fi.

@Zpmrk 

Your e4s will ONLY be limited to 100Mb if you connect them with ethernet cables. If they are wifi mesh connected to the S7 they will get between 3-500Mb/s depending on how far away they are from the main Deco.