Measure Lithium ion battery voltage (thus remaining capacity)
This seems to be very similar to Nick's schematic, was probably busy drawing it when he posted :-). 

First why you can't use the N-FET on the high side: it needs a gate voltage a few volts higher than the source, and the 4.2 V is all you have, nothing higher, so that won't work.

I have a higher value for the pull-up, though a value of 100 k&Omega; also will do. 10 k&Omega; will cause an unnecessary extra current of 400 &micro;A when you're measuring. Not the end of the world, but it's 1 resistor in both cases, so why not use a higher value.

For the MOSFETs, there are a variety of parts to choose from given the requirements are not so strict; you can consider inexpensive ones such as, e.g., Si2303 for the P-channel and BSS138 for the N-channel.