A few criteria:
Must be cheap
Must have replaceable battery
Needs to be android
Needs to be iPhone 4 sized.

Needs better specifications. 😉

iphone se 😀
Spend more money skinflint, or have you spent it all on a new bike? 😆

Don’t think any exist. You’d be far better off with a power pack than a replacement battery.

Sony Xperia Compact (pick model depending on price point).

There are numerous reasons why batteries aren’t user replaceable these days…

* Means turning phone off
* Can’t guarantee waterproofing
* Bigger batteries
* Can’t ship spares by post
* Risk of fire from unprotected batteries

I don’t know how big an iPhone 4 is.

Moto E4 is all of those apart from the iphone 4 size, so not quite what you’re after.

Moto e4 plus. Battery is not replaceable but I’ve gone from charging daily at 5pm to twice a week.

Moto E4 battery is replaceable and is possibly smaller than the plus – still bigger than IP4

you can get a small power bank that can charge phones while out on a big ride.

Check this out, GSMArena do what looks like a comprehensive comparison tool. Leading me to; this

For me a replaceable battery means that I can run a larger battery while just making my phone a bit thicker.
I dont want to carry another battery and cord around. Those battery pack covers for iPhones just make them not fit in your pocket.
I feel I may have to stock up on galaxy s5s .