that would be a size P crochet hook
A U.S. size 7 yarn hook is 4.5 mm. http://www.crochetmagazine.com/hook_sizes.php
It depends on the size of yarn used. For worsted weight size 4 yarn, it reccommends to use a size g or h hook. Check the yarn cover to see which size hook is reccommended for the yarn.
it is best to use an H or I size hook
In UK sizes, a 2mm crochet hook is a 14, it doesn't not have a US size equivalent. Here is a chart with crochet hook conversions from metric to US to UK.
A 5.0mm would be an H-8 hook.
A 3.5mm crotchet hook is usually an E sized hook. A G sized hook has the size of 4.0mm. Following is the set of seven size of crochet: E-3.5mm, G6-4.0mm, G7-4.5mm, H-5.0mm, I-5.5mm, J-6.0mm and K-6.5mm.
A 0.9 mm steel hook is about a US size 13.
If it's a bulky yarn then you should use hook size H and up. If it's thinner then stick to the smaller size hooks. If your hook is too small then the yarn will not fit on the hook and it may snag on the hook as your working.
A size 5 steel hook. http://www.crochetmagazine.com/hook_sizes.php
A 3mm crochet hook is halfway between a C and D size in US sizes.
For yarn, use an "E" or an "F".