is Dell Vostro 3500 Laptop a good laptop for 3 to 4 hours of online work daily?
Do you mean on battery life or plugged in? What type of work are you looking to do? What configuration of the laptop are you looking at?
Generally speaking though, even the base configuration is enough for using Microsoft Office and most web based applications:
Frankly speaking, the Core i3 processor and integrated graphics will be enough for many mobile small business users or students that are just looking to use productivity applications such as Microsoft Office and using the web to get work done. If you’re into gaming, multimedia production and design or tons of multi-tasking then you can certainly benefit from a more powerful configuration. Using the Core i3 and just 3GB of RAM I was satisfied with the performance of the Vostro 3500 for everyday tasks. In addition, HD Video playback using online sites such as Hulu.com or YouTube.com was smooth and flawless thanks to the processing power. While the Core i3 isn’t close to being the most powerful new Intel processor, I think it offers plenty enough power for mainstream users.
how would you compare vostro 3500 or 3400 with those of HP probook series 4320 and 4430 ?
I haven't gotten to use either series of laptops. However, I have had far more luck with Dell laptops than I have with HP laptops in the past. Dell's Business Support is generally better than their Home support, too.
edit: the Dell's seem like they are less expensive compared to the HP's. The base Dell Vostro 3400 gets you a faster CPU and an extra gig of memory for almost $100 less. You lose a little bit of HDD space, but the upgrade for that shouldn't cost all that much.