Welcome to Flyertalk and to the BA forum Acephalgic, it's good to see new faces here and you should make yourself at home here.

Landside from T4 to T5 is particularly tedious, so you best go airside on the bus service via the purple Flight Connection channel, which is well advertised in T4. It's a bit of a walk at the T4 end, but the official Minimum Connection Time is 90 minutes if on one ticket, and it's do-able in 45 minutes. You would need to allow at least an hour to do it landside. So in theory you have the time, but it will always be a risk on separate tickets: if your EK is late then you may risk losing the single flight a day to INV. You couldn't realistically look to BA to help you either in that scenario. If you're OK with that risk then go ahead, since the alternative is either going to LGW or going via ABZ. I guess if you did go for ABZ you could have a wider choice of flights to select, and if did miss your booked ABZ service BA might be helpful and allow you go on a later service.