It is very easy.
- Just install Binutils in N900
- Open texteditor (I use nano) and write code
- Assemble with as-command
- Link with ld-command
- set execute rights with chmod
- run (remember put ./ before your program name)

But I have question why
mov r7,#4
looks with hexeditor like this:
hex 0407A0E3=bin 0000 0100 0000 0111 1010 0000 1110 0001
but If I look ARM instructions manual, it should look like
1111 0i10 0100 imm4 0imm3 Rd imm8
or
1111 0i00 010s 1111 0imm3 Rd imm8?

And why there is in executable file 74 bytes before first command come? And lot bytes after commands and data?