It would be nice to think it's just the fuse, but on further thought it could also be the fuel pump relay, one of the most common 240 failure points. Fuse 4 gets its power from the fuel pump (system) relay. In addition to the O2 sensor heater, fuse 4 also powers the in-tank pre-pump, which can also affect acceleration under the sudden extra fuel demand. On spec and without further diagnosis, I'd now want to re-flow the main solder points in the fuel pump relay as well as cleaning up fuse 4 then make sure the in-tank fuel pump is purring by listening down the tank filler neck to confirm operation.

The '89 240 wiring schematics can be found on the same Google drive that I linked to. That '93 green manual is the only one I've recently spotted online for an LH 2.4 240. The wiring harnesses changed very little in the final years, with the exception of wiring added for things like ABS and SRS. Phil's '89 240 DL is the same year as the one I got brand new for my parents. I worked on it for many years. My brother now has it and lives in Phil's region of BC. It may well be the lowest mileage 240 in the entire province, not even 65K miles on it.

At least 240 fuse blocks are fairly easy to work on, starting with spinning the fuses to wear through any aluminum oxidation, pulling the fuses to make sure the pointed ends haven't worn through (that's a 240 gotcha that many people take time to learn), polishing the contacts and tweaking them in a bit if needed, even removing the spade connectors for cleaning the side terminals (not recommened unless you need to as it can weaken the connector, which may then need tweaking with pliers to get a good snug fit). Even removing the side panel and pulling the fuse block to service or even replace it is easier than pulling out a 700/900 relay tray so you can flip it over to access the back side.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now