Hi there N900 fans!

Today I found a battery originaly from nokia c6-00, to be more accurate the BL-4J.

Im using my nokia a lot lately out of home with no possibilty to charge so Im in need of spare batteries, so here comes the question:
Can I use the mentioned battery for n900? The size is the same, so the voltage as the original BL-5J (3.7V) but there are few specifications differences like below:

The original N900 battery:
1320mAh and 4.9Wh
But the battery I want to use as a spare one have:
1200mAh and 4.4Wh

I did a quick check, n900 boots up and working just fine, but will this not kill my nokia if I use it for loner usage?

In my opinion the BL-4J battery have just less capacity, but Im not a specialist so I rather ask, than fry my device.

Thanks in advance!